I dont know if you are a new player or an old player trying out a new class. Here's some tips for tanking in general.

-You are meant to take on the biggest, meanest enemy, not the whole group. Certain classes have really good battlefield control like dk talons and sorc encase but Temp and nb are really good at resource management.

-Resource management is key to your success. You can't just taunt the boss and block forever. Maintaining a bit of the damage and mitigating incoming dangers are much more important than trying to run around being a headless chicken(I tend to do that still but hey, im learning)

-Tweak your builds for multiple encounters. Dont get stuck with that one 'awesome possum' build thinking it will get through all the encounters. Different bosses have different mechanics that require different toolset. You can always use a hammer to cut down a tree but it aint gonna be pretty.

-Templar wise: Repentance is your friend. Blazing shield, circle of protection and Rune focus are really good tanking tools. Think paladin type of tanking and you are not far from emulating it.

Have fun mate and good luck.