You will probably lose a LOT. Nothing but tons of calistenics, and cardio. I wouldn't recommend trying to bulk up before, as they will strip it all off of you. They want lean athletic killers, not muscle heads. Even if you are ripped, they may make you a diet recruit if you are overwieght, and you don't really get to eat or drink crap. Physically it really isn't that tough.
Beyond that...remember its all just a big mind fuck. If you want to get punished but in return get promoted, go for squad leader or guide. If you want to go through as easily as possible, do everything right, and just try to blend in. Hehe, I still remember it like it was yesterday..."socks on now, socks off now, socks on now, socks off now" heheh just to fuck with your head. You'll see. =]