I'm a (relatively) new DM with about 20 hours of DM experience under my belt and I was wondering what are some frequent, common, or useful Tier 1 spells I ought to know well. Just wanna be prepared 😁

Suggestion, it is quite common, make sure to understand how it works.

I find that Charm Person, Shield, and Sleep pops up all the time. In particular you want to remember that Charm Person is not a "the fight ends" spell, unless the party is only fighting a single person (or more with higher lvl slots) and the rest of the party halts their aggression too. On the other hand, Sleep is a magically deep slumber so the targets don't wake up easily but it is not unbreakable. As for Shield it's just one of the best defensive spells there is, unless you're one of those evil DM's who won't reveal what the attacker rolled to hit. (Edit: I had indeed misread the trigger when I made this post.)

Dissonant Whispers is really strong.  Good damage, psychic damage which very few enemies are resistant to, wastes enemy reaction, and forces enemy to run if they fail their saving throw, but it uses their reaction and their movement so it allows opportunity attacks.  Combine that with a 7th lvl vengeance pally with tunnel fighter fighting style and the pally can follow the enemy their entire movement getting an opportunity attack every 5 feet.

Fire Bolt and Magic Missile are greatly effective offensive magic, no matter what level you are ... Considering you can also cast MM as a second level spell to increase it's potency.

Spells, ever new Dm's nightmare as they quickly change the game around and quickly end hours of planning. Remember to pay attention to casting times, material costs, and the components of spells, as players have a habit of forgetting these things. Most low level spells, while having material components, don't have costs associated, so it is usually not a big deal. But if the party ends up handcuffed, well, that could be an issue. Also pay attention to what spells are reactions and which are not. It may be important to remember that certain spells are bonus actions so a ranger couldn't cast Hunter's Mark and make a second attack with an off hand in the same round. Keep track of concentration spells. Your players will probably forget that the spell they used is concentration or "forget" to remind you.

So without further ado, spells to look out for:

At low levels burning hands is a strong spell. At 3d6 damage, it can possibly do up to 18 points of damage and deals damage in a 15 foot cone. This one spell can make any boss fight less of a challenge, and is hard to plan for since if the spellcaster uses it in an earlier battle, you may end up with a battle too strong for the party if you made the encounter harder to account for it.

Charm Person and Comprehend Languages are two spells that can make what you thought was going to be a challenge into a five second encounter. Though Comprehend Languages is less of an issue since you probably didn't know what languages the PCs were going to select before hand, but as the game goes on, it is something to look out for. Remember if you are in combat with a creature, the creature gets advantage on resisting charm person. Also note that charmed person doesn't allow the charmed individual to attack you, but form what I can see doesn't specifically state it can't attack your friends, but your friends can't attack it...so at least I think it has been a tradition of DMs to not allow the charmed creature to attack the casters friends either. Keep in mind that the Charmed creature is under no obligation to fight for the charmer, and so may leave the battle or continue to help their allies. Essentially the charmed creature can't attack the charmer and the charmer gains advantage on any ability check to interact socially with the creature. Beyond that it doesn't force any actions on the part of the charmed creature. Pay attention to the command spell as well as it specifically states it fails if the creature doesn't understand your language or your command is directly harmful to it.

Detect Evil and Good doesn't actually detect evil and good any more. Its probably the most confusing spell in 5th edition as in previous editions the spell actually detected good and evil. So a lot of people will try to use the spell to detect evil, primarily because that is what it has always been used for and it takes a bit to get use to the spell completely not doing anything it did before or its name would hint at doing. Protection from Evil and Good has the same oddity about it as the detect spell does.

Pay attention to the details of Disguise Self. Particularly what you can and cannot change. And remember the spell can not hold up to physical inspection.

Floating Disk is a cool spell to have, as it allows an easy way to carry heavy things or fallen companions. But remember, the disk only moves if you move more than 20 feet away from it and it follows you. It also has limits as it can't cross elevation greater than 10 ft. or more. The spell can't be used to carry the caster of the spell.

Hunter's Mark is a bonus action. This means you can't make an attack with an off hand on the round you cast hunter's mark or rounds you move it.

Mage Armor is an interesting spell. A protective force surrounds the caster. According to Crawford and sage advice, this protective force is not explicitly said to be visible or not visible. He allows casters choice. I'd probably go with it being invisible myself because that works more in the player's favor and allows the caster to keep the spell up while sneaking rather than waste it.

Shield. The fine print is important which twooshort missed. Shield is a reaction, and the fine print says "which you take when you are hit by an attack or targeted by the magic missile spell." This means you know for certain whether the attack hits or not, and also appears that you can't use shield if the attack misses. It also lasts until your next turn and so doesn't just help against the triggering attack.

Enhance ability helps with checks, nothing more.

Invisibility is a common spell. Remember it ends when an attack is made and doesn't rule out the possibility of the creature being hit by area spells or someone capable of hearing where the pc is.

Tiny Hut can ruin any midnight encounters you have planned. And really mess with your plans if it is up when the PCs are attacked as they can keep running in and out of it. And can still see what is going on.

Zone of truth prevents the target from lying. This doesn't mean the person can't say something false, provided they honestly believe it to be true. Doesn't mean they can't leave details out either or refuse to answer the question at all.

Counterspell is just something you should be aware of, especially if you have plans for a caster to suddenly escape or something.  

Fireball is awesome. That spell can ruin your encounter faster than almost anything else. You can be a jerk and always assume that if the caster doesn't mention otherwise that they are casting the spell centered on the creature they intend to hit, but for the sake of everyone else at the table, I beg you to ask the caster where they are aiming the spell, and then if they hit half the party that is on them.

Fly, it just changes the nature of the game and allows for some game breaking possibilities. Such as the ranger now flying above the fight shooting arrows at monsters who can't reach her any more.

Speak with dead and speak with plants can ruin any surprises you may have. I say ruin, but really if a player thinks to do it, it is not something that has ruined anything, except maybe something you wanted to reveal later. If the wrong thing is revealed it does have a habit of maybe derailing an adventure.

My best advice is to ask the players for what spells they chose for the classes that get to chose, and read over the other classes spell lists to become familiar with the spells. Alternatively you can wait and see what spells the players tend to use and learn those spells at first, rather than becoming familiar with everything all at once. I've tried to include spells that I think are common and might have some confusion/typical issues with. Doesn't mean I've exhausted anything though.

You may also want to approve any spell that occurs in a book other than the players handbook, and make sure you have an actual copy of that spell.

Just to point out, you can cast shield as a reaction after being hit by an attack. So you know for certain the attack hit. Even though you may not know whether or not shield will benefit you from that attack. It still lasts until your next round, so it isn't necessarily wasted.

I make my PCs keep track of the spells themselves. When they want to cast it they need to know everything about it. (Casting time, components such as Voice or materials, effect) I ask them the same every time:

"Who are you targeting? What components does it require? Are your hands free/do you have your focus out? What does the spell do? Is it a concentration spell? How long?"

I find that making the players keep track of the spells and who they're affecting frees me up to roleplay the villians better and keeps the players attention better.

Good advice.  I have a bookkeeping suggestion:   (Coming from an old DM with ~40 years experience)

I have been experimenting with this site at the table!  And last Friday night, I did a sanity check and went back to the old way.  So, I use a chromebook at the table, with lots of tabs.  3-4 tabs have the monsters open, and a couple of tabs each for magic items and spells.  Don't have to have the PHB, DMG, and PHB balanced and stacked where I knock them over.  And it saves me so much time!!!! 

Other suggestion.  Frequently the question from the player is some form of, can I do this with the spell?    the player has the book open -- ask questions.  ask casting time, ask them to read you the exact sentence, etc.  It serves two great purposes.  First, you might get enough to correctly make the ruling.   Second, it teaches the players what the spells can and cannot do and helps them make future decisions better.  

Finally,  I have seen Critical Roll episodes where both the players and Matt Mercer did not really understand the spell a character cast.  They simply worked through it at the table, everyone learned, and the world did not end.  I have had times in the last year where I have made a decision at the table, then after the session ended, went back, reread the rules, and simply approached the player out of game and said, hey it was a mistake, this is how it will happen going forward.  

Quote from NightsLastHero >>

Spells, ever new Dm's nightmare as they quickly change the game around and quickly end hours of planning.

I think some of the problems with Critical Role is that its possible they are getting spells mixed up from Pathfinder with 5th edition as they converted after two years of playing pathfinder. 

Matt also has a habit of not checking casting times when the spell negatively affects the players due to Keyleth's creative uses of spells. 

Hold Person is a good one that can easily melt bosses 

One of the best pieces of advice I heard from a much wiser DM was to include an NPC gatekeeper for magic users. If they're a wizard, maybe they are an apprentice to a skilled mage. If they're a cleric, they have to report to the temple of their god/goddess and meet with the high priest. A sorcerer might learn new spells from a dragon or Fey. This way the DM has some control over which spells are available to learn for the player. If you're worried about Fly or Fireball or Speak with Dead possibly ruining what you have planned and you're not ready to deal with those spells yet, having the magic gatekeepers not offer those spells to your players is an organic way to save yourself a lot of headaches.

On paper this method feels restrictive; but in actual gameplay it makes the world feel more alive and true to reality, as magic is a complex force to reckon with and for your PCs to just learn it on the fly as they level up feels very gamey. 

Quote from Type_303 >>

One of the best pieces of advice I heard from a much wiser DM was to include an NPC gatekeeper for magic users. If they're a wizard, maybe they are an apprentice to a skilled mage. If they're a cleric, they have to report to the temple of their god/goddess and meet with the high priest. A sorcerer might learn new spells from a dragon or Fey. This way the DM has some control over which spells are available to learn for the player. If you're worried about Fly or Fireball or Speak with Dead possibly ruining what you have planned and you're not ready to deal with those spells yet, having the magic gatekeepers not offer those spells to your players is an organic way to save yourself a lot of headaches.

On paper this method feels restrictive; but in actual gameplay it makes the world feel more alive and true to reality, as magic is a complex force to reckon with and for your PCs to just learn it on the fly as they level up feels very gamey. 

Some decisions like these you would want to make in advance, but I've played in campaigns where certain spells just simply weren't available. Speak With Dead comes to mind--a world where the death god had a lot of power and simply didn't allow the dead to communicate with the material plane after they died. (Raise Dead spells worked, iirc, but put you on that god's sh!t list :) Those sort of things need to be communicated with the players, but if none of them have had access to those spells before, you might just need to tell them when they level up and finally get access.

Another point to remember--Speak With X spells are going to be much more limited than some people realize. If you're speaking with a plant, it's still a plant. "How many orcs passed this way in the last day?" (How well can plants count?) A lot. Okay. "How long ago did they pass?" (How well can plants tell time?) Before today. Hmmm. "Were they carrying two halflings?" (How much can a plant really detect about a band of passing orcs? Plants can't see.) No idea.

Speak with animals would be more useful, but it's not like you're going to be getting a detailed report on the contents of a passing wagon from a bird. "What was in the back of the wagons?" Boxes. "What was in the boxes?" Metal things.  "How long ago did they pass?" Many days.

Speak with Dead's description clearly limits it. The dead creature can't speculate about the future, only knows what it knew in life, but it's answers are cryptic, brief, repetitive. And it doesn't have to answer if it doesn't like you :) So it's really easy to give the players useful info from their spell, but still not completely ruin a planned encounter. You really won't get much from interrogating the corpse of an enemy, and the corpse of someone not an enemy just may not know much, or be able to communicate it. "Who was it who killed you?" Someone in a red cloak. "Which way did they go?" Guy killed me, Mal. Killed me with a sword. "Did you see their face? What did he look like?" Guy killed me, Mal. Killed me with a sword. "Did you ever see him before that?" Saw lots of guys in red cloaks.

To be clear, the casting of these spells should usually give the caster some information. I mean, they can be wasted too, depending on what questions are asked. For example, if you ask a fern what color cloak someone who passed by was wearing, the fern may just say "What is 'color'?" But if used well and refereed well, they can help the players, without ruining your planning. 

Plants may have a better sense of time them you might think. Many plants actually turn to follow the sun during the day. Other plants use day length to determine when to leaf out, flower, and drop their leaves it the fall. Trees keep a record of what year things happened to them in their rings.

I would have them express things in a different way including how they express time. The important things in their world would include sunlight, rain, temperature, humidity, fire, and herbivores. 

A tree by a trail might not have taken any notice of a band of orcs passing by, but the grass on the trail could say how it was trampled. 

I would at least have the plants give as much or more information than a competent ranger could discover without magic but it would be a little cryptic. 

Quote from Trirhabda >>

Plants may have a better sense of time them you might think. Many plants actually turn to follow the sun during the day. Other plants use day length to determine when to leaf out, flower, and drop their leaves it the fall. Trees keep a record of what year things happened to them in their rings.

I would have them express things in a different way including how they express time. The important things in their world would include sunlight, rain, temperature, humidity, fire, and herbivores. 

A tree by a trail might not have taken any notice of a band of orcs passing by, but the grass on the trail could say how it was trampled. 

I would at least have the plants give as much or more information than a competent ranger could discover without magic but it would be a little cryptic. 

I definitely agree that different plants should know more than others. The idea that trees would know more is pretty classic and cool and makes a lot of sense. :) The notion of a 'memory' in their rings is also a good one, I like it. 

But a stand of swamp grasses doesn't seem like a good candidate for the same treatment. The rings on the tree lend themselves to counting. The grass to me has an idea of night and day, clearly, But counting nights and days accurately seems less plausible. 

The plants also can't move. The bush might know how the orc was moving generally when it passed the bush, but an exact direction seems implausible. A ranger is going to be able to follow tracks for a distance, well beyond what a plant would be able to do. The ranger can also look at both a sign on the bush and a mark on the rock wall above and tell something like the height of who he's tracking that the bush would not know. A ranger should be much more valuable for tracking (if he makes his rolls :), even in that exact moment, than speak with plants. That's the ranger's jam--equaling one of a party member's main skills with a 3rd level spell doesn't sit too right for me. But ymmv. 

That said, maybe 'grass' should be treated as an entire field of grass, given the intertwining of the root systems. So talking to 'the grass' may actually get you a direct path of whoever walked over it. But even then, the grass wouldn't know what they looked like, how tall they were, what they were carrying, etc. I completely agree that, if a smart player has a whole forest of plants to talk to, he could get a lot out of it. But I'm going to put that burden on my players. If they just ask a tree "which way did they go", the tree isn't going to be able to help them much. 

One of my favorites is Faerie Fire. Everyone in your party can attack targets with advantage when they fail to save vs. this spell, which makes it a real game-changer. Bards, Light Clerics, Druids, and Archfey Warlocks can all get this one at first level.

i was thinking about what the ranger could get from the plant only. Direction of travel, how long ago a twig was broken etc. A ranger putting together many pieces of information form different sources would be able to a lot more than one plant could say.

I imagine the using the spell and asking “which way did they go?” and the the plants in the area all swaying in one direction like a breeze blowing along the orcs path. Every plant the orcs touched would bend in that direction.

Quote from Trirhabda >>

i was thinking about what the ranger could get from the plant only. Direction of travel, how long ago a twig was broken etc. A ranger putting together many pieces of information form different sources would be able to a lot more than one plant could say.

I imagine the using the spell and asking “which way did they go?” and the the plants in the area all swaying in one direction like a breeze blowing along the orcs path. Every plant the orcs touched would bend in that direction.

Love that visual :)  What I forget as a player myself is that the spell lasts 10 minutes. That's a lot of asking questions of a lot of plants, if the player thinks about it, asks the right plants, and the right questions. In my experience, most players (me included as a player) ask one plant, get what they want or not, and then move on. 

 I haven't found Speak with X to be game breaking. The target can only say what they know, and may lie or give incomplete answers. It also depends on what the players ask.

Magic Missile is one to watch. Those darts don't do a lot of damage, but they always hit. They are very potent against concentration spells...

Detect Evil and Good has changed substantially, but Protection from Evil and Good still functions as it always did, to keep out magical or enchanted good and evil creatures. As long as you keep in mind that it is supernatural good and evil you should be ok with these.

Charm Person is not mind control. The target regards the caster as "a friendly acquaintance", they are more suggestible, but are not slaves.

Sleep has a duration of only one minute, or ten rounds. When the spell ends the targets awaken. You can use it to sneak past the guards if you are quick, but don't expect them to still be sleeping when you come back.

The slowing effect of Ray of Frost is easy to overlook. Ten feet may not seem like a lot of movement, but it is one-third of a standard human's move. It can be enough to keep an enemy from reaching you this turn.

Make yourself a crib sheet of the spells your players have. Pay special attention to range, and duration as these are two very important aspects of spells that put limits on what magic can do.  Make crib sheets for monsters and spell casters you plan to send against your players. It is worth jotting down the key aspects of spells that you might overlook in the heat of play.