I'm not going to repeat what has been said so far.

I'll add that Youtube is your friend with great GM sharing their knowledge about the rules, gameplay, adventure book, suggestion, finding inspiration and others.  I'm looking at the Dungeon Dudes, Ginnydi (She got cool NPCs), Taking20 and HowtobeagreatGM so far but i know there are plenty of others.

The description will be something that the players will remember when done right and this goes both for NPC and Areas.  You want to make them unique (use 4 of the 5 senses, taste might not be an option) without dragging too long into descriptions... Who wouldn't remember eyepatch Bob who tried to backstab one of the PC or Gum-Gum the goblin showing her big smile as the PC turns a corner.

Be opened-minded to receive comments from your players to make the game better and become a better GM.  In other words, don't take it personal and use your instinct to determine how you can use their concerns to improve the game.

Have some standby one shots you can tailor to suit the occasion . Your party will go off on a tangent from your story line at some point, so something handy to fill the gap. 

Be prepared to be derailed-it happens a lot. 

Start with the base idea of a campaign, and work your way backwards towards where the party currently is. I've designed the final dungeons for campaigns within the first few sessions, and it worked out very well. Flesh out the story from the finish to the start, and your players will have lots of fun while you tailor the stuff much later on to their liking.

Another useful nugget of DM wisdom a friend told me is that wherever the party is going, try to conveniently set up plot points in that general direction. If your ice castle is to the east, but everyone wants to go west, then you can have some other major plot point to the west as well. Don't do this to the extent that the party can't escape the main story line at all *ie don't move the ice castle to in front of wherever the party is going*, but do it just enough that the option is there to hop on the main line wherever they are.

1. get a feel for the room. If you feel like the pace is too fast slow it down, if the pace is dragging on a bit pick it up a bit. And if your players are just sitting around roleplaying and having a good time let them take the pace at their own rate and join in on the fun with an NPC or two.

2. If you think you have enough random NPC's you don't, add more. Flooding cities with NPC's really brings the game to life. players will never know if an NPC is truly important to the game or not and it really makes the world around them feel real with every action that they take (Make sure to remind the party that they will never know if NPC's will truly bring them value to the campaign or not that way they don't go off running down every rabbit hole you throw at them and they really think about what path they want to take)

3. Make sure that a natural 1 is a fail no matter what a player is attempting to do. players have a tendency to think that they are invincible and with some of the smart players stacking stats create a  "floor" for their ability checks that can become nuts. Players need to remember that even the greatest most powerful creatures still mess up and fail from time to time.

1. You're there to help tell a cool story with your friends. Let them have their say in how it goes.
2. Encourage and reward "bold choices".
3. Don't roll for stats.

1.  You must become proficient in the art/science of making and presenting dungeons in your game.  It is important to think about dungeons in a broad way- a dungeon is any finite, discrete, and bounded area containing various obstacles and challenges where the game of D&D takes place.  You can’t have a D&D game without a dungeon. 

2.  Mastery of combat is key.  Being able to provide varied and interesting combat that is appropriately challenging is something every DM should strive towards.  To accomplish this use a balanced mix of Physically tough monsters, no-so-tough monsters that compensate by being fast or sneaky or good archers, spell casting monsters, and uniquely challenging monsters (like an intellect devourer) in your encounters while also varying the terrain where the battle takes place. 

3.  Game management skills and people skills are important.  Ensure no player is being left out, apply the rules fairly, pace the game appropriately, be respectful and reasonably accommodating to the players, and be receptive to feedback without taking offense. 

I guarantee these are the 3 most important things for your D&D game.  The story could be vague or the NPCs forgettable.  That’s ok, and something you can work on after you master these. 

1) If you use NPCs in combat, keep them simple, and let your players control them unless absolutely necessary...no player likes to "watch" a combat between the DM and the DM.

2) Take PC decisions into account with the overall story...if they decide to spend two weeks on a side quest, let them do it! but make sure that decision has an impact on your overall story (what does the BBEG do with that two weeks?)

1.If the players see a otter they will want it as a pet .2.its ok  to use humor 3.Cats will knock over mins and wreck things if you don't shut the door.

My top three?  Wellllllllll...

1. Always, ALWAYS, look out for your players.  There have been countless times as a player where other players at the table have bullied and/or ignored and/or belittled me, and I have noticed the same patterns developing in my D&D sessions with me as DM.  This is especially prominent in West Marches style campaigns.  It is up to you as the DM to make sure all players are being included and are having fun, especially those with quieter voices or who are shy.  As a DM, as a friend, as a human being, you MUST make sure that all your players are comfortable - even one bad D&D session can potentially scar a mind for months and sometimes years.  It's a basic human right to feel safe, especially in a simple game such as this.
   
   
2. Don't prep.  Try improvising instead.  This may sound counter-intuitive, and might be hard for some, but I have found some of my best sessions have occurred by me abandoning my campaign script and winging it based on an idea I or one of my players have had.  This method of DMing is unbelievably difficult to get a hang of, and its okay to only use it once or twice over the course of a campaign, but it can lead to some of the most precious storytelling moments you will ever experience.
   
   
3. Accept that not everything will be perfect.  You can plot a session for weeks, with an amazing dream in mind, and have it fail catastrophically.  Your new campaign might fail to start with the bang you had planned, or you might accidentally have your main boss of the adventure killed early on.  These can seem like monumental mistakes, but in truth they are often smaller than they appear to be.  The trick is to pick yourself back up, learn from the mistakes made, and try to improve.  Even experts can muck things up, but it takes a true wise DM to learn from them.

No 1 - Don't ask for pointless rolls. If I'm not willing for their yo be a consequence to failing, I don't ask. If I need to reveal something key to moving the story along, I don't ask. If most people could do what the player is asking while drunk, I don't ask.

No 2 - Tie in character backstories. We ask players to write like a page of backstory and they take pains over describing their motivations and the key people in their old lives. Why ignore that effort? Plus the players get a kick out of realising it's their turn to be the star of the show for a bit.

No 3 - If you have time, make something. I know we don't always have time with all the other things to sort out and you know life. If you can write out an incriminating letter or a series of recurring hieroglyphs that are important the player get excited about it. Suddenly they are discussing it at length in stead of making notes while I read it out.

1) Players are people first. Treat them accordingly.

2) Always have a plan.

3) Always be ready to scrap the plan.

1. Don't plan to far in advance.  Players have a way of messing up your plans.  They may not want to go in the direction step by step that you want.

2. Come to each session fully prepared.  Have all pre-ordained encounters ready 100%.  Know your monsters. For players it can be frustrating to sit there and watch the DM try to figure out what is coming next (also part of not planning to far in advance as you may forget what you planned) and or figuring out monsters.

3. Plan for what some call In Town time.  Many players like to occasionally have an in town day for doing things outside of adventuring.  When they spring it on  you that this is what they want to do you wheels may start spinning.  I set up a number of buildings and street encounter with a list of random things that may happen. 

Quote from bobberuchi >>

1. Don't plan to far in advance.  Players have a way of messing up your plans.  They may not want to go in the direction step by step that you want.

2. Come to each session fully prepared.  Have all pre-ordained encounters ready 100%.  Know your monsters. For players it can be frustrating to sit there and watch the DM try to figure out what is coming next (also part of not planning to far in advance as you may forget what you planned) and or figuring out monsters.

3. Plan for what some call In Town time.  Many players like to occasionally have an in town day for doing things outside of adventuring.  When they spring it on  you that this is what they want to do you wheels may start spinning.  I set up a number of buildings and street encounter with a list of random things that may happen. 

I'd like to add one thing to this list about preparing. 

Have a list of names. Just a bunch of proper names to give your NPCs. 

If they go into a town and bump into someone and you name them "ummmm. Ummmm. His name is. Uhm Bob." They know that you are making it all up on the spot. 

But having a good name really helps in creating the illusion that you totally have it all figured out. 😂 Truth is the DM rarely does, but the players don't need to know that. 

So inspired by your list, I'll add a thing to my original list

1. When improvising, try not to give it away. Prepare some generic stuff to help create this illusion and then build up from that. Some of my most memorable NPCs started as just names on a paper.😅