I started DMing in my own custom world to begin with. Its kind of freeing because you can just make EVERYTHING up and not have to worry about existing lore or cannon. I few things I would suggest is do a session 0 where you talk about your campaign and place the characters into the world. You can finalize building characters in the day and talk about the world that you are all jumping into.

Before your sessions 0 communicate out what you inspiration might be including specific references to media if applicable. For my first campaign it was "generic forgetten realms / dnd lore with more planar holes". Some other campaign pitches I have done are "cowboys and aliens", "U.S revolutionary war as seen by Hamilton in Dnd", "The Seige" and "Young world with big monsters". I am currently playing in a dnd campaign that was pitch as "animals as characters in a is redwall meets, Princess Mononoke meeting breathe of the wild campaign".

So keep it simple, give specific references to inspiration or theme if appliciable.

For the first session have a single hook and a single winnable combat ready. That is all you really need.