how do u prep for dnd i feel like i should be doing that before sessions
That's a great question! And it varies from GM to GM. If you're new to prep, it can be a little overwhelming - start with ONE thing you want to focus on (materials, scenery, voices, combat, lore) and play around with that; figure out your own preferences. I'll try to be comprehensive [aka i tend to ramble - I'll also try to format it so it's not just a wall of text.] If you already are confident about some of this stuff, great! But I hope some of it helps.
In my opinion, prep is as much about feeling prepared and confident as it is about having materials ready. Both are important! TL:DR; it's mostly about PRACTICE and ORGANIZATION.
I'm a person who tends to run a bit of a sandbox world, and trust that I'm prepared to improv a little when the players inevitably go 'off course'. But I have a lot of GM friends who are much further towards the "I like to have everything prepared, with a set map and a set understanding of IF you do this, THEN this will happen."
It can be a good idea to be honest with your players about what to expect - especially if you have new players, making sure they are prepared may be part of the process. Letting them know whether there will be combat, and whether there will be time in-game to prepare for it, can help a game run smoother. (I'll come back to that.)
Know how to use whatever resource you are using. If you are running a game, whether physically or digitally, figure out how you like to set up your area so that you know everything you need is within reach. If it is in person, make sure things like books and maps are all in once place ahead of time. If you are calling with Discord, you could make sure the server has a dice bot, or make sure any music bot you are using works properly. If you use roll20, google drive, or another site, you can upload maps, make sure they are aligned to the grid if they need to be, and play around with any other features it might have! There are some useful tools out there for GMs, and even just finding something to help you organize your notes can be huge. That could be a whole separate post.
If you're running a game from a module/pre-written materials, you want to spend the time reading through those materials. It's honestly a bit like studying in a class! Make sure you know some of the information, and know where to find the rest of it.
For familiarizing - whether I'm using a guide or writing it myself - I try to prioritize visual and narrative descriptions - practice narrating what your players 'see' when they enter an area, for example, so that you are prepared for any potentially unfamiliar or confusing words (I'm a pretty confident reader and I still stumble over things like "minute" [pronounced min-it] as in 60 seconds of time being spelled the same as "minute" [pronounced my-noot] as in tiny.
This goes for NPC voices too. If important plot information is being delivered via dialogue, say your line a couple times! If you are giving your characters specific voices, practice how you want their speech to sound! (I like to take notes for that too - this guy sounds "comfortable and friendly, like a dad at a barbecue". This woman sounds "like a librarian - pursed lips!" It can help to find a key phrase, gesture, or posture that helps you get into character. I really enjoy doing NPCs, so I tend to start with that, but not every campaign has a heavy focus on them.
Keywords can help you give 'character' to your environments, too. This foggy coast might be ominous, so I want to narrate it in an ominous voice. This tavern may be comforting - it's warm, so I'll narrate it in a warmer tone. Listening to your own voice ahead of time can help you set the mood you want a scene to have.
You may also want to try to anticipate player questions. This gets easier with experience. If you have a wizard who likes to look for spell components, expect "could I find a sprig of mistletoe?" when the narrative takes them to the forest. If you have a rogue who really leaned into the thief archetype, expect "is anything on the table small enough to fit in my pocket?" If they encounter a dog, no matter how menacing, expect that someone is probably going to ask if they can try to befriend it. Knowing what your players enjoy, and what their goals are, can give you a better sense of how to anticipate their responses to what you throw at them.
If there is going to be combat, puzzles, or obstacles, have stat blocks you may need available. If they are in a book/ on a web page, BOOKMARK THEM. If not, write them down!
I have found it very useful to get note cards, and write a 'cheat-sheet' with some basic stats down from my players' character sheets. My current version (for D&D - other systems have other stats) includes their max HP, AC, passive Perception, base stats, languages, and their main weapon - anything I might otherwise have to ask them about, I like to have in front of me. It's not uncommon for a GM to say "What's your AC?" when they roll an attack against a PC, but I often have newer players, and I like being able to say "You're using a short sword - that's 1d6 of damage, plus your strength is +2" right away.
Even if your players are experienced, knowing what they are capable of as their GM helps you write challenges that they are appropriately equipped to solve. If your party has a lot of spell casters, maybe give them a puzzle about solving runes to get into a locked chamber. If there are a lot of fighters and barbarians, maybe the barrier is clearing a rock slide, or scaling a wall, or fighting a guard. Give the players chances to show off the things their characters are good at!
I also make note cards for any monsters in combat, and use my note cards to keep track of initiative order in combat, by putting them in an ordered pile - that way I have the stats for whoever's turn it is right in front of me when their turn comes up. But that's just my personal preference, and many GMs I know will use an online initiative tracker (roll20 has one built in!)
Having stats ready and accessible is so helpful. For some GMs, (and some players!) being familiar with the combat they have planned IS what prep is all about! Even if you're a more narrative-focused GM, read through monster stats in advance - I've run some combats that were harder than I realized because I went "ooh, fire mephits sound cool! I could put some fire enemies in an abandoned forge" and didn't consider the fact that they also explode when they die and damage anything within 5ft of them...
and that most of my players were melee fighters.
Oops! Lesson learned. Which brings me back to working with your players and their abilities.
One of the simplest things you can do to prep is to remind your players that the game is coming up, and to make sure they've done anything they need to do. Just a "Hey, we're playing tomorrow. Do you have any questions for me?" Being approachable to your players helps you get to know them, and in turn helps you anticipate what they will respond to. Plus, asking "Would you like any help leveling up your character? If not, may I see what new things you chose when you leveled up?" when relevant gives you an opportunity to get more familiar with their character sheets.
I hope that helps! If you have any other questions, if I can elaborate on anything here, or if there is another GM-ing related topic you would like my thoughts about, let me know!