"hey, I want to run a game about {theme, conflict} in {setting} using {rules system}. You want in?"

When you have enough interested players, do a "session 0". Talk about what kind of game you're aiming for (serious? Comedy? Deadly? Power fantasy?). Set lines and veils. You're going to have a bad time if you're aiming for tactical combat in a grim world, but two of your players want Loony Toons.

I highly recommend players make character concepts together. You should probably play "you all meet in a tavern" once or twice because that's kind of a whole Thing, but that brings in a bunch of problems.

Read how Fate does it: https://fate-srd.com/fate-core/phase-trio . Maybe just read the whole section on starting a game there. A common property of good DMs is reading other sources and stealing from them.

Absolutely do not get super committed to any particular story beat. Do not expect the players to get captured, or for them to agree to a sea voyage. If you want the game to be about something like that, start there. Tell the players. You can absolutely pitch a game based on "you were all overcome by an orc raising party. You'll start as prisoners planning an escape". Do not play out them losing the fight.

Your players are probably not going to pay a lot of attention to your setting. This is doubly true if you unilaterally made it up. If you give players input into the setting details (see again how fate does it), they'll be more invested and remember more. (Though some players hate having narrative agency, so watch out for that)