I'm so jealous, I wanted to go to that but couldn't find anyone to go with me.
Anyway, you'll meet plenty of interesting characters in the mosh pit. There'll be a middle aged bald beef guy who'll simply go around swinging his elbows and fists. He has no mercy, avoid this guy. There's sure to also be a really high guy with dreadlocks and he'll swish them about whipping everyone, also avoid him because when people push behind you and you get your face squished into his hair, ropeburn. When I saw megadeth there was a fat sweaty chick red and crying hysterically in the mosh pit beating her fists on this one guy's back. If you're lucky you'll see that kinda thing, always funny.
Mosh pits aren't too bad, I'm a 5ft1 miniature female and I'm usually okay. If you fall over people will pick you up. I've always avoided the wall of death though. It's always fairly easy to push yourself out if you don't wanna mosh anymore. Either way just have fun. Make sure to listen to Darkest Hour, their guitarwork is really great. Machine Head are superb as well. Personally I'm not a fan of the other two.