Believe it or not, there are DOs and DON'Ts to working out at the gym.  For example, you don't answer your phone when you lift weights -- leave it in your locker.  You don't want to be the guy talking on his blue tooth as he looks at himself in the mirror while doing curls.  As for workout apparel, it's probably best to dress conservatively, so you don't get awkward stares the entire time.  Male booty shorts and Army boots?  Uh, I guess only if you're big enough to back it up when somebody tells you that you look ridiculous.