A lot of players go in without much understanding of how LFG work, and they make a lot of mistakes in their role and in common behavior. With that in mind, I've compiled a few pointers for certain roles and for dungeon divers in general:
-Communicate. I emphasized that word because it is so often ignored in PuGs. Silent dungeon runs are awkward, clumsy, and often catastrophic. Talk to your party when you need help, when you have a question or comments, or just to lighten the mood. When parties communicate, they work better and they have fun.
Pay attention to /p and /s chat. Players can and will speak up if something is wrong or if they need something. Having the channel disabled or the chat window hidden is a poor excuse for a mishap.
-Need versus greed is a very serious matter, and should be respected without exception. If an item is up for roll, and it benefits you directly (your class, your stats, or your profession), you have the right to roll need. However, it is unfair and uncool to roll need on something that you don't actually need, potentially robbing another player of something they could use. Don't be surprised if you get vote-kicked for being a loot ninja. No one likes loot ninjas.
-Try your best to have fun. That's what most everyone is here to do. If a pull goes badly, if the group wipes, or if something else isn't quite right, don't get worked up. Raging , flaming, and quitting in a huff isn't fun for anyone.
Specific tips for DPS (Damage Dealers) :
-Contrary to popular belief, you actually have two main jobs: deal damage, and don't make things difficult for the tank and healer. Stealing aggro, pulling extra mobs, and running ahead or away from the rest of the group makes you more trouble than you're worth. Watch your threat (preferably with an AddOn like Omen) and watch which mobs you attack.
-It's typically the tank's job to pull, especially in classic 5-man dungeons. Regardless of whether or not you can get to the mobs first, it's harder for the tank to get the aggro off of you then it is for him to just pull.
-Aggro on anything besides the tank is bad, but mobs hitting the healer is especially troublesome. While not really mandatory, a superstar DPS will make an effort to take aggro off of the healer right away. If you see someone wailing on the priest, target that bad boy right away and start wailing back. The tank can take the aggro off of you as soon as he's ready.
I hope this helps.