1- DO NOT HURT YOURSELF. Any exercising you do, take it easy, stretch as necessary, if it hurts badly- DON'T DO IT.
2- See #1. Seriously.
3- You will get plenty of exercise at BCT. But if you want to start now, run and walk some, do some pushups (see youtube videos on how to do them properly) and situps (again, don't strain anything- go easy and on a padded surface..)
Your first week at BCT you will need to run a mile in 10min or less and like 10 pushups and 10-20 situps. Then the real fun begins.
So run a mile or two without stopping a few times per week, more if you are used to running.
But again- SEE RULE #1!
If you stress your body out now you'll be hurting at Basic.