10) Download Add-ons
There are a large amount of WoW add-ons which can make performing your role much easier. Some add-ons even help you rate your damage output (or healing output) versus other members of your group. My background is mainly in PVE so I will speak to a few of the common ones just as an example. Deadly Boss Mods (DBM) is an extremely helpful tool which assists in drawing your attention to certain key points of the fight. It will give you countdown timers to the most important parts of the fight (the boss is getting ready to enrage, you need to interrupt the boss soon, etc). If you raid, you will absolutely want this add-on.
Damage or healing counters are also very important for measuring how good you are doing at DPS or healing. How can you improve or see that you are improving if you aren’t measuring your DPS or healing? This can also help you identify other players who are your spec that are doing better than you so that you can ask them for advice.