Have you had the indent all your life? If you have, it's probably pectus excavatum. This is a result of the ribs growing inward instead of outwards, and you should have a doctor look at it, especially if you're still growing.
I myself have it (but I'm a girl, and it tends to be worse with guys) and I've had to get EKGs, sonograms on my heart, breathing tests, etc, to make sure the ribs aren't putting any restraint on essential organs. Luckily, mine is fine and doesn't need to be fixed. It can be fixed through two different surgeries, but they're very invasive and IMO not worth it unless it's a health risk. Although the indent is noticeable when I wear a bathing suit, I don't find it necessary to do anything about it.
And if this is the case, surgery is the only way to fix it. Since it's the way your bones are, no amount of working out will fix it.
Here's some more information about it: http://en.wikipedia.org/wiki/Pectus_excavatum
Hope that helped!!