JAredl888,
I suspect you are still fairly young, say 17, 18 or 19, right?
The two things that matter most for infantry is stamina and strength. But these are not related to weight. If you are in good physical shape (and I suspect you are), consult your closest recruiting centre to obtain the physical standards you will be expecting to meet at the time you join (the phys ed. test, whatever they call it nowadays). Test yourself against it and if you can pass it, don't let your weight stop you from joining. You will bulk up in time, like any other young athlete does from his early twenties to mid twenties.
If you really are concerned with that, talk to a trainer at a good gym in your vicinity to go on a weight lifting program and muscle building diet (but remember in the end you are not looking to turn out like Arnold Schwartzeneger, Jesse Ventura or "Rambo") - supplement, yes if applicable, but no growth hormone or other medical trickery.