If you're not sure about something, don't be afraid to ask. It's better to find out ahead of time than to make a costly mistake during an encounter.
Don't talk to NPCs or activate anything unless the raid leader has said to do so. Some events must be timed from the start of a trigger and sometimes a specific person needs to trigger the event.
Do not cast area-of-effect (AoE) spells near sheep or monsters that have been immobilized by a crowd-control spell (CC).
Be kind to your group mates. They are the ones who will be best able to watch your back.
If you become separated from the raid party and aggro the surrounding monsters, it's often better to die where you are rather than attempt to run away. Another raid member can resurrect you.
If there is a task that you're expected to accomplish but are unable to do so, let the leaders know. They may have some ideas on how you can overcome your obstacles or have alternate activities you can help with.
Don't attack a target until told. Usually the main tank has to generate aggro on a given target before it is safe for other players to attack.