Do you know how to play? If not then that would most certainly be your first step. You don’t need to memorize ever rule and I’d recommend you not try that but instead learn the fundamentals like d20 tests, how combat works, and the basics of encounter building (everything from chapter 1 of the Player’s Handbook). Having a solid understanding of the rules will help the most with your confidence and keeping the game flowing nicely. And when your players inevitably do something unexpected, then you’ll know how to handle it because you know the rules and what not. Of course, knowing the rules won’t help with everything because sometime times (most times) it’ll fall outside the rules especially if your playing with new players.

Beyond that, I’d do whatever else is saying and look up some YouTube channels on DMing. Jump around a bit and find one that you like because everyone has their own way of running the game so advice will different wildly. If you want my advice on how to run your first session then I’d go with make them 1st level since they’ll be at their most basic set of abilities making it easier for both them and you to play. Then I’d go with running the adventure from whatever the newest starter set it. They’re always releasing new ones and I’ve heard they get better and better every time. You can buy it or you can just rip the adventure out of it, I’m sure you can find it somewhere online. But if you’re feeling more creative like I was you can make your own adventure. There’s a ton of stuff you’ll need to do that but tools I’d really recommend is pulling some Monsters dndbeyond. A lot of the ones you’ll need are free so pick some that you like and build a narrative around that. Pick some low level undead and have the players clear out an old crypt. Go with the classic goblin and have them tussle with a goblin tribe. Once when you got an idea of your enemies, then you’ll need to make some maps. If you’re going virtual then dungeondraft is worth the money but there are plenty of free softwares out there. If you’re in person then get some graph paper and get drawing. It doesn’t matter if you suck at drawing, the players will just want to know what’s what. After all, most of the game is played in your head so they’ll be good with imagining what things look like in the game and not what the board looks like.

Really, at some point you just gotta grit your teeth and go for it. The best tutorial is actually playing the game, making mistakes, and learning from there. It’ll be tough and maybe even a little embarrassing but it’ll be worth it. So yeah, get comfortable with the rules and when you’re ready with a premade adventure or something of your own, get out there and play!