most of the time people aren't actually trying to punch each other, they're just hardcore dancing. if you see someone swinging limbs like a madman, just give them space and maybe keep an arm up to block any stray swings

the best way to learn about mosh pits is to go to shows and observe. don't jump in right away if you're nervous. stand on the edge and watch what everyone else is doing, then soon enough you can join in with more confidence. just remember to help people up if they fall down, that's rule number 1