You should always prepare yourself for basic so it will be easier overall. While others are collapsing you'll be fit and in shape. This isn't a way to be cocky, but a way to show that you know how to prepare for things and push yourself already.
Pushups, situps, and running. Look at FM 21-20 for approved exercises you will most likely be doing in basic training, and do em. It'll help you a lot if you know the exercise, know the regulation on it, and can do it relatively well.
Go past exertion, it'll be the way you'll be doing it in basic, and it will prepare you far better than not going past exertion.
My favorite quote, that I don't know the author of sadly... "Reach your peak, then go past it."