Congrats on the enlistment!
First of all, don't try to kill yourself! You will have plenty of time for that at Basic.
If you exercise start off slow and build up.
3 months is Plenty of time! Anything you can do- running, walking, any cardio, plus any crunches or situps or pushups- will help get you into shape. You'll still feel the pain of Basic, but yes it will be easier.
There are plenty of magazines or programs online, look for "100 pushups" or all the different plans to run a 5k or 10k.. The Physical Fitness tests in the Army are pushups, situps, and 2mile run, but don't limit yourself to just those.
Others will have good links or better advice- Good luck though!
Be Proud of what you do- and do it Well! ~me