Im leaving for the Military in a couple months and the only thing Im worried about in BMT is the pushups. I can barely do 20, I've been practicing alot but I cant see myself doing the amount required. I also find myself pulling something in each shoulder, do I need to work out my shoulders more? My MAIN question is, what GNC product would you reccommend to strengthen my shoulders or myself in general?

EDIT: not just to take alone, I mean along with my daily gym shedule.