Congratulations! I'd recommend wearing something comfortable because you will probably be there pretty much all day. Please wear underwear... I can't believe I have to say this but when I went to MEPS for my physical there were 2 guys in the inspection room, out of about 20 guys in the room, that did not wear underwear and thus were not allowed to participate in the ortho screenings so they were sent home to come back at a future date. Trust me, nobody wants to go through MEPS more times than absolutely needed.
Be sure to get plenty of rest the night before and just relax.