Memorize your General Orders, and how to report your post, you can also help yourself out by not asking questions just doing EXACTLY what you're told. Physically if you have passed MEPS you'll be fine, the PT at boot camp will get you in shape if you're not, and if you are all ready in shape you'll be more in shape it's designed that way. If you are weak you get to go to another platoon to do remedial PT until you're in shape enough to keep up, this will however extend your stay at the Depot. Being mentally ready is probably the number one thing you'll want to be. Make sure you tie up all loose ends on your personal side, that way you can have a clear head. 3mo is a not a long time (it seems that way but it's not), and when you're done you'll look back and sometimes (only sometimes) wish you could go back.
UPDATE
Ignore Nickleback15 he is a poser. The Marines will teach you to fire a weapon properly, they will get you in shape, they will teach you martial arts based on the MCMAP program (Marine Corps Martial Arts Program), and it's not sign language it's hand and arm signals. I HATE POSERS!!
UPDATED AGAIN
Man, that guy is an idiot. JUMPING FROM HELICOPTERS????? The closest you'll get is a tower, and it's down ropes not wire. Also you'll do the gas chamber, but you'll have a gas mask, it's done with CS tabs, not gas bombs or whatever....I'd sure like to meet that guy.....