I race my own boat and have crewed on a bunch.
Good advice so far....
Be prepared mentally for a little yelling. Be prepared mentally for a little cursing. Directed at you! Be prepared for yelling between the foredeck and the cockpit crews.
I try to never raise my voice at my crew. I've been sworn at like you wouldn't believe on some boats.
Just be prepared for it. At the start, at a mark rounding, dousing or gybing the spinnaker, heavy traffic, crossing situations, things can get tense and exciting. Just remember, no one means it. The whole crew is under stress, that's all.
Also DON'T talk to other boats, yell "Starboard" or something. That is up to the skipper.
Be early. If the skipper says everyone at the dock at 10, be there at 9:45. Be the first to arrive and the last to leave.
Help with everything getting the boat ready and putting her away.
I usually like to answer when given an order. For example, if asked to ease the jib sheet, reply "easing the jib sheet".
This is very constructive advice. I particularly like the advice about not talking to other boats. I have been shocked by some of the inappropriate exchanges that have gone on between boats and I certainly don't want to develop that reputation for my boat as, ultimately, it is the skipper who answers for the conduct fo his/her crew.
Also, don't underemphasize the importance of promptness. One of my least experienced crew is always early and he gets Big bonus points, as I do not like changing sails or doing other prep work all by myself. One of my more experienced guys is severely time-challenged, requires "reminder" calls an hour or so before the race starts and I finally had to announce a rule that we would be off the mooring at a specific time because we got so sick of waiting.
If you work your way into the regular rotation, make sure you give as much notice as possible when you can't make it, as it there is nothing more maddening than finding out that you are short-handed at the last minute and trying to cover.
Finally, some degree of yelling may be inevitable, but if you get on a boat where you are being abused, try your best to find another boat. Life is too short and it's supposed to be fun.