First is, get the class stuff out of your head. Classes only add the flavor of combat tactics to TESO. You dont hit harder because you are a Dragonknight or regen magicka faster because you are a sorcerer, no. The class skills mostly all do the basic of things... damage and crowd control, which dragonknight and Templar have tanking and healing skills respectively.
If you are tanking, you wear heavy, if you need stamina and/or stealthing, you need medium, and if you are using class skills like a beast, light armor. I am not gonna get into the 5/2 split, as there are other guides out there.