Use a tiny RPG system[1] with a tiny environment ready-made environment[2].
Familiarise yourself with how others[3] run the same environment, then make it your own! You got this.
[1]: System (free). https://yochaigal.itch.io/cairn
[2]: Environment (free). https://v6p9d9t4.ssl.hwcdn.net/html/6466170/BotEk/index.html
[3]: An actual play of the environment (free). https://www.youtube.com/watch?v=e3N4pqHIEwQ
Follow
@evan I spent many hours thinking about how to answer your question, here's the story of how I distilled this answer: https://gist.github.com/cognivore/cdadb983a335d17cf1dd1c3b578486cf
(and three bonus suggestions from my experience in mentoring an actual beginner GM).
@jonn This is a great distilled piece of advice.