GTA is suppose to be up now, but when I checked about 11am it wasn't. Expect an update before playing and possibly learning tutorials before you can do much.
Rockstar has a breakdown of what you can do so far, heists and other features will slowly come out.
Remember to sign up for a Rockstar Games Social Club account and then join the TTDILA Crew.
Join us so we can take Los Santos for all it's worth! You also get special items for being part of the Rockstar Games Social Club.
Update: Los Angeles I'm Yours has another comparison of famous places of LA you can find in Los Santos. I like there photography of the Los Santos map you get with disc copies of the game.