I was lucky to find a group that's really casual and friendly, and they're great about helping new folks learn. (And just all around fun to ride with!)

So my knowledge is totally casual :

Let folks know you're new and unsure of yourself, and that you'd appreciate hints.

Know and use your signals, especially for slowing down and stuff. Look before you turn.

Warn someone if you're drafting off them.

Don't overlap tires; if you're not sure, a little extra distance in front of you will help.