We don't know what GTA 6's launch-day experience will be like. We do know Rockstar's history with recent launches, and what to look for.
This is an editorial read, not gameplay strategy. For the broader regions, see the regions explainer.
What the in-game map will probably look like
Mostly fogged. Rockstar's latest titles (GTA V, RDR2) begin with the map concealed save for a few locations and gradually unlock it as you travel to new areas. Expect Vice City to be partially unlocked at the start (story-related), with the remainder of Leonida fogged out.
The game will unlock new areas as you progress through the story. By the end of Act 1 you should have most of Vice City unlocked, and by the end of the game, the entire map. The pace varies, though.
Probable starting areas
Speculation. Most Rockstar games begin with the main characters in a very limited setting - GTA V began mid-heist in the snow. Evidence from the trailer suggests Lucia and Jason's journey could start inside, or very near, a prison, before they make their way out into Vice City.
It's a safe bet the first playable area will be somewhere in Vice City or immediately around it. The swamp/wetlands and the Keys analogue are likely later-game reveals.
First-day priorities (editorial picks)
Drive the bridge route to the Keys when it opens. Fly a helicopter over Vice City to scope out the layout. Check out the art deco strip at night. None of this is necessary - they're just the things most worth doing first.
What to be patient about
The full map takes time to unlock. Login/server problems - GTA Online had a rocky start. Some content (multiplayer, online modes) will likely come after singleplayer instead of at release.
Do not expect everything to work perfectly on day one. Rockstar releases are messy.