Unless it's one of the newer theaters, it's going to be much smaller than you think. Watch your head when going down stairs! Bring whatever your friend will like (remember most dressing rooms are tiny, so overly fragrant flowers can be annoying- and you don't know whom they may share a room with).
In general: Stay out of people's way, don't touch anything (you'd be surprised how many people will just pick up props/costumes without asking- don't do it, no matter how tempting), don't take photos of anyone without asking, and most of all, don't overstay your welcome. But have fun!
"What- and quit show business?" - the guy shoveling elephant shit at the circus.
