pushups, situps, run, run, run, run... repeat. You'll be a running machine by the end of basic (assuming you're talking the army or marines).
Another good exercise to learn is "front back goes", basically the DI will yell front and you jump down to the ground as fast as you can and start doing pushups, then he'll yell back, and you flip to your back as fast as you can and start doing butterfly leg lifts, then he'll yell goooooo, and you get up as fast as possible and start running in place. they fuck with you to, sometimes not giving you enough time to flip over or jump on the ground before they yell the next command, so get real good at that.
Also, practice doing half pushups, what i mean is, they'll make you get in front leaning rest position (big word for push up position) and then yell half way up, and you come half way up and hold it, then they yell half way down, and... well you get the picture.
Basically any exercise that helps with endurance and agility will help you. The hardest part of boot camp is standing in formation for hours at a time and staying awake though the lectures. The pt really isnt that hard after the first week or two and of course getting to throw grenades and shit is lots of fun
