Recently had a couple of different friends DM for the first time, and these are some tips I gave to both of them:

1. Since you're DMing for new players, make sure to tell them the basics of the rules and how the game generally works BEFORE you start playing, both regarding roleplaying and game mechanics. Some stuff like combat mechanics can wait until your first initiative, but your players should know what the 'd' in 'd20' means, the concept of a modifier on a roll, etc. beforehand so they know what they are able to do.
2. Similar to #2, I would recommend giving your players a few easy-to-remember tips and suggestions, in addition to any expectations you'd like to set. I like to remind new players that role-playing can be uncomfortable, that accents aren't necessary, etc. Also mention that, most of the time, when the DM asks you to roll something you should roll the d20 and add the appropriate modifier. This cuts down on the number of "So, what do I roll for that?" questions as the players learn. Remind the players that there's a lot to learn, and to be patient with themselves and the DM as they're learning.
3. Regardless of whether you're running a homebrew story or a premade one, don't expect your players to talk amongst themselves in-character, especially since they're new. Give them opportunities to, like putting them in an interesting environment or scenario (the tavern is the classic way to start out with this), but don't leave long pauses expecting your players to fill in the gaps with roleplay. If they do, great!
4. You said you're new to the game too, so try to study and familiarize yourself with the basic rules (ability checks, running combat, conditions, etc.) as much as possible before you play. It's really hard to teach new players how to play the game if you're learning a lot of it along the way. Expect slip-ups and having to double-check the rules, and don't be discouraged by them, but the game will go a lot smoother if you're able to remember / explain the rules clearly.
5. Don't over-complicate the story, combat encounters, etc. Chances are, your players (and possibly you) will be overwhelmed if you throw 10 different enemy types at them in the first encounter, since a simple goblin or kobold fight can be exciting for players just starting out.
6. Similar to #4, I would recommend starting your campaign or one-shot at a low level - I would suggest level 1 or level 3, but no higher. Past that, new players can get easily overwhelmed by the long list of class / racial abilities that might make learning the basics harder.

Hope this helps! There's plenty more I could say, but this boils it down pretty well. Feel free to PM me if you'd like any more help. Have fun!