You're in the same boat as I am: 5'8", 29 yrs old and 147 lbs. As someone suggested earlier, P90X works if you really stick to the "no pain, no gain" mantra.
Also, if you don't want to spend that kind of money and are dead serious about getting in top-shape, I suggest that you try either of the following:
1. Cross Fit www.crossfit.com
2. Body Rock @ www.bodyrock.tv
I alternate between the two.
Mind you, I also watch what I eat, but since you're 18, you don't have to.
Edit: By same boat, I meant back when I was your age. Now, I'm just fit. :P