As a new dungeon master I am having a hard time figuring out what monsters I should put in my campaign. Can someone please help me figure out what to do?
Not a lot of context here. Maybe start with your setting, level of characters etc. Are they in the wilderness, mountains, city, town? In the mountains you would use Kobolts, in the wilderness, goblins and in a dungeon or catacombs use skeletons/zombies.
All of these would dictate what monsters to use. Then go to the encounter builder and if the characters are level 1 or 2, filter the monsters to CR 2 and see what comes up. Then you have your pick to the monster that fits the situation. Figure out the situation and you figure out the monsters.
Consider the themes and setting of your campaign, and then see what monsters fit that. Some books include what terrain the monsters can be found in, which is incredibly useful for ensuring the party aren't just running into the same types of creatures regardless of what terrain they go to. I'm particularly fond of Kobold Press' Tome of Beasts (1-3) and their Creature Codex (the first Tome of Beasts book didn't have the creatures listed by terrain, but the others do).
For Tier 1 (Levels 1-4), your party are local heroes, likely fighting beasts and bandits. Depending on your preferences, you might introduce goblins, kobolds, or undead, but consider that the earlier the party fights a type of creature, the more likely they'll consider that creature fodder later, so if you want something like undead to be a big deal, I'd suggest not using zombies or skeletons at all, and when they finally come across undead, have them be higher CR undead so it's more of a credible threat and the party has to treat them as such. For later tiers depending on the themes you may focus more on monstrosities, aberrations, etc, especially since the higher level the party gets, the fewer humanoids in the world there should be to match their level, due to most people not doing anything that would cause them to gain character levels, so humanoid opponents that are credible threats should typically get more rare, unless your setting specifically is like an MMO in which a lot of people reach high level.
For something like Dragons, if you want the mythos of them to remain intact, and them not just be prey for your mighty adventurers to hunt, You might make their first encounter with a Dragon an environmental encounter to be survived, not fought. Maybe they're in a city and a Red Dragon Greatwyrm flies over destroying the city, and the party have to try to save who they can as they escape the city through various skill checks, and possibly battling through Kobolds to escape. Then maybe later they hear rumors of a blue dragon in the desert that recently laid an egg, so the party might have a stealth mission to steal the egg, or destroy it before it hatches, since the Adult or Ancient blue dragon is still above their pay grade. Then late game they can have the opportunity to target adult or ancient dragons directly. But remember the action economy and how much damage your party can dish out, and don't forget the dragons Legendary Actions, Lair Actions, possibly Mythic Actions, as well as its small army of Kobolds with siege weaponry to harrass the party and even out the action economy, as well as being there to break concentration on debilitating spells that might otherwise take the Dragon out of the fight and make it go down like a chump.
Start out figuring out what monsters you think are cool. Flip through the monster manual, look at the art, skim the descriptions. Chances are you'll find something that intrigues you, and it shouldn't be hard to come up with an in-universe reason for why the players might need to fight one.
To get "technical" I think you're talking about encounter building. If you have the DMG, you may want to review that section, and if you don't, or even if you do, you may way to play around with Encounter Builder tool on this site.
That said, if you're really at a loss or feel trepidation to building your own adventure, there are published adventures, some freely available online. Reading through some of those may give you a sense of how to build a D&D adventure. I think any of the recent anthology books put out for 5e, Radiant Citadel, Candlekeep Mysteries, Tales from the Golden Vault might be useful. Lost Mines of Phandelver is free here, not an anthology but a good example of how a campaign works, with escalating challenges, etc.
Without more info i'd try to go by terrain or climate type such as artic, coastal, desert, dungeon, grassland, mountain, hill, forest, Underdarkn, urban, swamp, sea or underwater and then look for list of Monster By Environement typically found there. When i design encounters, i also consider if it's a random encounter or one that is not randomly happening establish what is the purposes and finally i set the difficulty i want it to be, either easy, medium, hard or deadly.
As a new dungeon master I am having a hard time figuring out what monsters I should put in my campaign. Can someone please help me figure out what to do?
Not a lot of context here.
Maybe start with your setting, level of characters etc. Are they in the wilderness, mountains, city, town? In the mountains you would use Kobolts, in the wilderness, goblins and in a dungeon or catacombs use skeletons/zombies.
All of these would dictate what monsters to use. Then go to the encounter builder and if the characters are level 1 or 2, filter the monsters to CR 2 and see what comes up. Then you have your pick to the monster that fits the situation. Figure out the situation and you figure out the monsters.
Hope that helps.
#OPENDND
Consider the themes and setting of your campaign, and then see what monsters fit that. Some books include what terrain the monsters can be found in, which is incredibly useful for ensuring the party aren't just running into the same types of creatures regardless of what terrain they go to. I'm particularly fond of Kobold Press' Tome of Beasts (1-3) and their Creature Codex (the first Tome of Beasts book didn't have the creatures listed by terrain, but the others do).
For Tier 1 (Levels 1-4), your party are local heroes, likely fighting beasts and bandits. Depending on your preferences, you might introduce goblins, kobolds, or undead, but consider that the earlier the party fights a type of creature, the more likely they'll consider that creature fodder later, so if you want something like undead to be a big deal, I'd suggest not using zombies or skeletons at all, and when they finally come across undead, have them be higher CR undead so it's more of a credible threat and the party has to treat them as such. For later tiers depending on the themes you may focus more on monstrosities, aberrations, etc, especially since the higher level the party gets, the fewer humanoids in the world there should be to match their level, due to most people not doing anything that would cause them to gain character levels, so humanoid opponents that are credible threats should typically get more rare, unless your setting specifically is like an MMO in which a lot of people reach high level.
For something like Dragons, if you want the mythos of them to remain intact, and them not just be prey for your mighty adventurers to hunt, You might make their first encounter with a Dragon an environmental encounter to be survived, not fought. Maybe they're in a city and a Red Dragon Greatwyrm flies over destroying the city, and the party have to try to save who they can as they escape the city through various skill checks, and possibly battling through Kobolds to escape. Then maybe later they hear rumors of a blue dragon in the desert that recently laid an egg, so the party might have a stealth mission to steal the egg, or destroy it before it hatches, since the Adult or Ancient blue dragon is still above their pay grade. Then late game they can have the opportunity to target adult or ancient dragons directly. But remember the action economy and how much damage your party can dish out, and don't forget the dragons Legendary Actions, Lair Actions, possibly Mythic Actions, as well as its small army of Kobolds with siege weaponry to harrass the party and even out the action economy, as well as being there to break concentration on debilitating spells that might otherwise take the Dragon out of the fight and make it go down like a chump.
Start out figuring out what monsters you think are cool. Flip through the monster manual, look at the art, skim the descriptions. Chances are you'll find something that intrigues you, and it shouldn't be hard to come up with an in-universe reason for why the players might need to fight one.
To get "technical" I think you're talking about encounter building. If you have the DMG, you may want to review that section, and if you don't, or even if you do, you may way to play around with Encounter Builder tool on this site.
That said, if you're really at a loss or feel trepidation to building your own adventure, there are published adventures, some freely available online. Reading through some of those may give you a sense of how to build a D&D adventure. I think any of the recent anthology books put out for 5e, Radiant Citadel, Candlekeep Mysteries, Tales from the Golden Vault might be useful. Lost Mines of Phandelver is free here, not an anthology but a good example of how a campaign works, with escalating challenges, etc.
Jander Sunstar is the thinking person's Drizzt, fight me.
Without more info i'd try to go by terrain or climate type such as artic, coastal, desert, dungeon, grassland, mountain, hill, forest, Underdarkn, urban, swamp, sea or underwater and then look for list of Monster By Environement typically found there. When i design encounters, i also consider if it's a random encounter or one that is not randomly happening establish what is the purposes and finally i set the difficulty i want it to be, either easy, medium, hard or deadly.
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Lost Mine of Phandelver - Lost Mine of Phandelver - Sources - D&D Beyond (dndbeyond.com)
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I think development of the Encounter Builder on DND Beyond has been abandoned as it has been in beta for over 3 years.
However, it is still a good tool for getting suggestions.