http://dulfy.net/2014/12/11/swtor-3-0-defense-guardian-tanking-guide-by-artorias/
this should get you going
edit: whoops that's an end game guide not exactly a leveling one. It does have a great explanation of abilities and cooldowns etc.
There is no real tank gear until late 30s or early 40s.
Mainly as a tank make sure the bad guys are attacking you and not the healer. Be aware of what is going on, put camera zoom at 100 so you can see whats going on.
Enable target of target so you know if you loose aggro
Never start a fight with a taunt
DPS should follow the weak to strong kill order