Make sure you wear goggles or glasses so you don't get the BBs in your eyes. I would recomend long pants and long sleaves but you don't need them they just absorb the shock of the shot a little. That's about it just goggles are really important. I love airsoft. Have fun!
If the guns you play with are high velocity wear a mouthguard. It's not hard to lose a tooth.
Make sure your goggles are high quality, ballistic approved. If they are weak, they can shatter on impact, and the shards will make the results of being hit in the eye ten times worse.