The principles are pretty much the same as in WOW, so if you came from tanking there you will already know the basics.... It will just be about learning the abilities your tanking class has to his or her disposal... There are plenty of information regarding ability usage, rotation and stat prio here on the forums for both tank jobs so that shouldn't be very difficult to dig up....

As for fights, your best bet is to watch quick mini tank guides on YouTube... These vary from 5-10min and basically gives you a heads up on tanking specific mechanics in any given dungeon/trial/whatever... I can personally recommend MTQcapture's videos as they are very good and strait to the point...

Also if you already have a lvl 50 melee character you probably already know some of the dungeons, you'll just be on the other side of the mobs, but mechanics are basically the same... The mini guides are really only if you are doing things for the first time and to keep you from running in blind... Most likely doing this as a tank will wipe the raid/dungeon/whatever unless you have FC members/friends that guide you through stuff...