Halo 3 isn't Bioshock. It doesn't have plasmids or a way for you to invent new items. You don't get to decide whether or not you're going to play it good and save the little sisters or harvest them. You're not trying to figure out who you can trust.
In Halo 3, you're going to war. In a very real sense, Bioshock and Halo 3 are different genres. Sure, they're both shooters, both FPS, which technically makes it seem as though they're the same, but … no.
Halo 3 fires up with a loading screen not unlike that of the beta – landscapes of blue and the exact same music. The entry page gives you an innocuous menu of choices: Campaign, Custom Games, Matchmaking, Forge, Theater. Just pick one. You'll be able to get to any of them later without going back out to the main menu. None of this requires great explanation, and you don't need to RTFM.