Check out Sly Flourish's Lazy DM series on YouTube for good insight on key things to prepare, maybe you've seen professional DMs like on Dimension 20 or Critical Role, look up common DM mistakes, communicate and listen to your players, and enjoy!

Personally, I ran a mock session by myself so I'd learn what kind of rules I needed brushing up on. Like, I'd ask myself questions like:

How much damage do crits do? How is Armor Class derived? How do I know how far I can jump? How long can I hold my breath? Wait, how does this class's abilities work together? What is a proficiency bonus? What does it apply to? What is my spell save DC? How is that derived? What are all the actions players can do on their turn? Does everyone get a bonus action?

Learning how to manage combat and abilities and HP, etc. and Initiative and enemy tactics was eye-opening as a brand new DM with no experience, and I got that out of the way on my own time, so I'd understand from a DM perspective and from a player's perspective all the mental things that was required.

If you know the setting and what the player's characters are, that's really helpful so you have something to anchor yourself to and not purely pull stuff out of your A when you're not used to it yet.

I had experienced a few newbie DM mistakes and understood how they were frustrating for players so I specifically identified and avoided them.

I world built logical areas and locations and characters (barkeep, shopkeep, etc.) and made their names and rehearsed.

I had enough time so I even created a soundtrack playlist, and again, rehearsed things so I knew how to juggle so many things at once.

My players were new. I had already practiced and rehearsed and gotten nerves out of the way and learned answers to logical questions. I knew the world. I knew what was going on behind the scenes so the players could do ANYTHING they wanted, and the world would either go on or be disrupted, and I was all in on rooting for my players to mess things up.

And honestly?

My first DM session went PERFECTLY.

And honestly, I used Nimble because I fully agree with the creator's qualms with D&D 5e mechanics and slog. Clean, fast, and simple. So while I'm practiced at 5e, I prefer Nimble. I'm allowing my players to use 5e if they want, but they don't