A mosh pit is an open area of the concert hall, where a section of the audience begins to move back and forth, then shoves, and soon they're shoving harder, slamming into each other. A line of people around the fringe stands shoulder to shoulder to keep the moshers within the area so nobody goes reeling into the ordinary concertgoers.
According to "A Guide to Moshing and Concert Survival," it doesn't matter much what you wear to mosh, so long as your clothes are baggy. Be sure to wear a belt. Wear shoes, not sandals or slippers. Feet get stepped on. You don't want to break other people's toes either, so ordinary sneakers are your best bet. Tie your laces in a double knot to prevent them from falling loose. Leave your wallet in the car; it might get stolen or lost if you carry it. Also leave your cell phone behind. You can't talk on it with the music playing, and it'll just get broken if you take it with you. You'll want to remove anything else that might hurt you when you fall on it, such as your key chain or coins. Keep only one car key, to get back into your car where you've stashed your stuff. String the key onto your shoelace, and tuck it into the laces to not lose it. Remember to tie that lace in a double knot over the key.
"Know your enemy," says "A Guide to Moshing." The risk involved in slamming varies according to the genre of music. Here are a few:
An ordinary alternative crowd is just there to have fun. There is little danger in a mosh pit at that sort of show.
At a punk show, the purpose is to hurt people. Have your guard up. Nobody will pick you up if you fall down.
Celtic is akin to punk, but more fun.
The most dangerous pit will be at a hardcore concert. If you're not hardcore, be very afraid.
Even so, actual punching is frowned on and standing your ground is thought a bad idea.