I am joining the army this summer, how do i know if i'm to "lean" for excersise? and i weigh 150 pounds i am 16

You're probably not: Almost no one as young as you is too thin to exercise. You will have a physical exam on induction. The biggest mistake you can make is to try to gain weight. It could lead to a lifetime of trying to lose it. On the other hand, it is the army. Stay well hydrated by noticing your urine output and how dry your mouth is. There is nothing like exercise to improve your mood and your attitude.

Don't know your: Height. If you think you are too lean to exercise then how do you expect to make it into the military. Recruit training requires physical exertion. Check standards to see if your weight meets enlistment standards: http://usmilitary.About.Com/od/joiningthemilitary/a/height.Htm.