Watch what other mages are doing. Learn from their mistakes, learn from their good plays.
Mages were a mystery to me too. After making friends with good ones and duelling them over and over (I main a DK) + experimenting on my own, I've managed to learn the class somewhat.
You can make a mage on Blackrock and look through the talents and spells thoroughly, get some idea what you can do with them.
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EDIT:
/agree with above. "That is what makes mages hard, not the skillcap but the skillfloor."
Mages really aren't that hard if you practice for a while. Like others have said, they have a higher skill floor than most classes. You can't spam 3 buttons like scrub paladins, and you don't have much of any survivability without good timing and making use of the proper armor etc. You need to react quickly to certain things, and know when to hold or use CDs properly or you will die without very much effort on the enemies part. Saving abilities like blink, using all of your spells like frost/fire ward, remove curse, changing armor mid fight depending on who's attacking you etc will let you use a mage far more effectively than most people on this server seem to be capable of. You need to have a deep understanding of every classes abilities in order to play one to it's full potential. A similar example would be rogues.