You want to run Dungeons & Dragons for the first time. Awesome! You’ll have a great time. Here’s some advice for the absolute minimum you need to do for your first session to make it run as smoothly as possible.
Ahead of Time
Find a first level adventure. While this will take some work, it’s a lot easier for a first-time DM than building your own scenario. The “Lost Mine of Phandelver” adventure in the D&D 5th Edition Starter Set is a great start, and there are quite a few more on dmsguild.com.
Read this adventure thoroughly, multiple times, before you play.
Read the Player’s Handbook or other core rules thoroughly. Don’t worry about memorizing every rule, but the more familiar you are with the rules, the better.
Find an encounter sheet (like this one), which has space for several monsters, plus an initiative tracker. Print out a bunch of copies and fill in several for the encounters you think you’ll have in the first session. That way, you’ll have all your monster stats and initiative on one sheet of paper.
Setup at the Table
In your notes, have a sheet of paper or text file for the players’ characters. Write down each PCs’ player name, character name, race, class, and passive Perception score on this sheet. This will be very useful to glance at as you play, so you can address people as their characters and handle passive Perception checks.
If somebody has a question about a rule and you don’t know the answer, make a quick “ruling.” This is a temporary decision about how that aspect of the game works. Don’t worry about looking it up unless somebody already has the Player’s Handbook open near that rule. If anybody’s concerned about that, choose a player who will write down the rules questions you all have as you play. Somebody can look those up afterwards.
Related to that: If a player doesn’t understand some aspect of their character, encourage the player to look it up in their spare time. In other words, if they chose the “Light” spell, but only wrote down “Light” on their character sheet, then in-game asks you how “Light” works, make a ruling, then tell them roughly where it is in the Player’s Handbook and have them look it up later. It’s not your job to know how the PCs’ spells and special abilities work.
When you’ve finished describing a scene, ask the players some variation on, “What do you do?” Maintain the focus on them and their actions. Keep encouraging them to make decisions and move the story forward.
Good luck! If there’s anything I missed, please post in the comments.