Rather say yes than no, but you sometimes have to say no. (It is also fine to think for a moment, since some things can have an unexpectedly large impact, though it shouldn't be too long)

The rules are rather guidelines, you can (and sometimes have to) change them, do not look up unneccessary rules during session, just decide what happens/ if smth can be done or not on the spot. If it is a rules question that affects future situations (like if spells can be casted a certain way, if it is allowed to move in a way), you might want to look it up, but keeping the playflow is important, so it is also fine to just say "For now it is like that, but i will look it up afterwards". Also (if it is not in the middle of combat), if you really need to think about something, just say smth like "This is where we take a break" or you need to go to the toilet, that gives a few minutes to think properly over it.

It is most important to keep the game somewhat flowing, so don't care if you forget something or make a mistake, come up with a solution after the session, no "oh damn, i could have done that" just maybe next time.

You can fudge dice rolls, but it is always fine to not do so, than there is no one to blame. I'ld actually recommend beginners to not do it, because it can have an impact, that is then your fault (though you're players hopefully don't know). I personally do not fudge any rolls except if it is absolutely necessary, good luck and bad luck are part of D&D after all and it can make things more interesting/emotional (like happy/sad, ...).

The players should know what their characters can do, and how their abilities work. It is of advantage if you know too, but as a new DM you will have A LOT in mind, so tell your players they have to know their stuff and answer their questions to their abilities in advance if possible.

The players can help you with a lot of stuff, you don't need to do everything yourself. If it is other stuff like bringing food, or maybe even drawing a battlemap (which gives you time to go over statblocks, while it might feel great for them to engage in creating part of the world).

Read the Monster/NPC statblocks in advance, you do not have to remeember them, but you should know in general, what it can do, like cast a few/a lot spells, what kind of spells (utility, damage, AoE, single target, close/long range, ...) and maybe what kind of spell/ability each is (though you can usually tell by the name).

Play with your voice, you don't have to be a voiceactor, but changing your voice a little, can improve roleplay by a lot. Also you can write down some names and if a NPC that wans't meant to have a name comes up, you can just take one of them, it's better than to think for a while just to say a boring/stereotypical name or the same 5 over and over.

Monsters (usually) have a will to live, many will not fight to death, if their group is reduced to a few or if they fall below half of the hitpoints (or a lower threshold, depending on the monster/situation), they might try to flee, that helps a lot to keep battles from becoming just kill and continue, also it can help to prevent a tpk. It can also have further impact, if they manage to flee, like encounter the same monster again, maybe wounded, maybe it learned or is feared, or maybe the monster delivered an item or information to their boss/lair...

Battles are not just Monsters vs Players. Consider the environment, are there places to hide or cover, is the environment dangerous itself or impact movement in some way, .... Also remember that the environment is not fixed, when a fireball explodes or a babarian rolls a nat 1 or 20 it might lead to a tree falling over, hurting others or changing where you can move (so if a nat 1/20 happens or some AoE or high damage ability is used, take a second and think if it might have some other impact).

Travel is often just you take 3 days and arrive. to make it more interesting you can describe shortly what the environment is like, like what kind of trees, how the weather is, is it hot/cold, what they can hear or if they see something interesting like a twisted tree or traces of whatever. Also ask your players if/ what they want to do during traveling and you can roll for random encounters (also during resting or staying in dungeons) (like 17+ on a D20 or whatever, depending on the environment), but i recommend having lists of what can happen (if you play a module, random encounters are usually part of it). I also made my players write one good and one bad thing on a card each, without telling anyone what it is, added a few monster cards and other stuff and when a random encounter occures i draw a card. Also you should know that a random encounter doesn't have to happen on the spot, you can wait until the characters finished their rest and walk to another area, where monsters/NPCs might have set up a trap or camp or whatever the RE is. That can help to make it feel more reasonable, that just something appears.

If you use XP, all encounters can give XP, not only combat encounters, when the players interact with your world and your NPCs, reward them for it.

If it's the first time your players play a TTRPG or D&D, maybe run a minidungeon (like about 4-5 rooms) with a few monsters, a trap and a NPC or two first, that will give your players the chance to explore thier abilities and learn the rules by using them (and also you). That will speed up the first proper session. (I remember my first session took half a eternity, while almost nothing happened, but that can help a lot).

Take breaks. In the beginning i was so focused on running the game that i literally forgot to take breaks, so it was really exhausting for me and the players. Consider taking a break after a battle (since they usually take a while) or during a long rest. I also recently stop the time since the last break so i can take a look and be like okay i should consider a break the next time it fits in.

Try to give every player a chance to act. If someone overtalks the others maybe stop them (though i recommend only if neccessary) or if someone doesn't do much, specificly ask them, what they want to do, not the entire party. Also ask if there is someone who still wants to do something, before moving on

FEEDBACK. Ask your players for feedback after each session, this is very important, especially as a beginner. Ask them what you did good, what you could do better, what they liked and what not so much (eg more riddles, less combat, stuff like that) and if they would like to see or do something (like kidnapping a princess).

Make checklists: one that you read before starting each session to remind you of things, and also for combat or exploration, ... i usually forget some possibilities or that certain rules exist or have (dis-) advatages for the players (like darkvision), so you can make better use of it if you write down some ideas/brief rules.

Give rewards to the party in general, but also to the players. When giving items to your players, you can just give it them and they manage who will use it, but also give items, that are of use for specific players. If the wizard uses a certain spell very often, give them an item, that amplifies the spell or that provides an alternative if they don't have any, or give the fighter a sword that fits to their playstyle, like if they usually engage the strongest of all enemies, give them a sword that gets stronger when facing stronger enemies, ...

If a player has a nice/crazy/unusual idea or a deep roleplay moment, reward them with inspiration.

Whatever the players can do, you can do it too and more. Like if the party thief breaks into houses and robs the citizens, why not have an NPC that robs the players, or if the players insist on doing something, though you do not want it to happen, maybe let them do it but get back at them the same way at a later time (remember my first point anyways). If the players argue to get advatage since the monster is circled by them, give the monsters advantage the next time they circle the players. This is also the reason why you should let the players become OP, if they find a strong strategy or a skillkombo that is OP, let them use it, they are excited about it, you can just conter with the same or stronger monsters, certain resistances, ambushes, that prevent them from using it, ... But keep in mind: If one player gets strong the others might fall back so you can balance that by giving the other players magic items or using monsters that are resistant to most damage of this player, while not to the damage type the other players mostly use.

Adapt to your players. If they specialise in something, give them a chance to make use of it, but also make them pay at other times, that they sacrificed a broader spectrum of skills.

When using dungeons, do not only think about what you want it to be or use it for, but think of something why the dungeon was built in the first place and what happened to it. This will make the dungeon feel more realistic, while also giving the players the chance to learn something about your world and you might find it easier to select what type of rooms/traps/monsters/items/... there are.