1.) Play your character against some test dummies/mobs. Use this time to try and learn every ability your class has, and what it does. At this time, put them all on your bar and figure everything out.
2.) Look up a (recent) guide online for your class.
3.) I suggest downloading dominos (an addon) at this point and setting up your abilities that you will be using. I highly suggest binding the majority of your abilities to keys or mouse buttons. (dont be a clicker! its too slow!)
I personally use a mouse with a standard wheel, buttons, and two extra side buttons. The way I bind my abilities to these are extra skills on the two side buttons, scroll up, scroll down, and clicking the scroll in. With those five buttons, I also bind shift + all five of them as other abilities. etc.
4.) Now that you have your bars set up and everything bound (which you can tweak as you go), follow the guide to dps, healing, or tanking priority and try to get it down well. This will only help you in the long run.
5.) Once you begin doing dungeons and raids, don't stop this process. Continually repeat it until you are utilizing all the little caveats of your class. Don't be afraid to look back at a guide and say... are there any abilities that I am under-utilizing?
6.) Grab some addons to make your life easier and your skill in that role much easier. Power auras makes me not have to watch my spirit trinket, so instead I get a giant icon telling me that my spirit trinket is ready to go. This makes me able to utilize my items and abilities to their maximum value, which can go a long way.
