As a newbie to the game, I've found that just saying "Hey, first time here" or somesuch has gotten good results in people being forgiving, and sometimes even explaining the fights. If you're DPS, you just hit things like normal. If you're a tank, you just try and hit everything. If you're a healer, keep the tank alive, but I doubt you're playing healer if you don't like being in a group.

And if you're talking about a Raid, in ARR as I discovered, mostly you'll just be hanging around in a group, and other people will mostly be killing things for you. It's a little frustrating actually, but you don't have to worry much about not doing anything.