I'm assuming you want to be in the Infantry, so I'll tailor the answer for that.
After Battle School you will be posted to a Battalion, if you are posted into a Light Battalion(3rd) your chances of getting into the Jump Coy are good, of course you have to be a good troop, PT, not be a s**t pump, etc. You can ask to go into the Coy, or it could rotate between companies so you can end up in it that way(I know 3VP does/did that), or because of re-org/manning you could be told you are going into the Coy.. If you are posted to a Mech Battalion, you might be able to get the course, but unless you get posted to a Light BN you won't be getting into a Jump Coy. Nothing wrong with being in a non Jump Coy either, still has excellent soldiers, and opportunities to do some good training and jobs.
Honestly at this point in time, why are you even thinking about this? You're 15 and have quite a ways to go before you can even join, let alone worry about getting into a Jump Coy.
