You upgrade equipment by either Improving it at a Crafting Station or by using something that is more on your level, which in your case will be L6 or L7.

Howwever in the leveling stages, loot drops are the best source for new gear, as it does not not cost you anything, and you get experience getting the stuff by killing the mobs.

Casters are best in Light Armor, but while leveling, you want to wear a mix of armors to level each of them up. This way if they change the game mechanics, you can just change the armor and respec, not grind out armor skills. Once you get armor leveled enough to unlock the 5 piece passives (around Armor L35 or so) then go 5 Light/2 Heavy or 5 L/1 M/1 H

Templars have issues with resource management, so the Seducer 5 piece set would be good to get later on. Couple that with Cost Reduction Jewelery or Regen Jewelery and another complimentary 3 peice set and you will be good to go with magic resources. I would not worry about Crafted Sets until the L30's or better, unless you can already make them for yourself. Seducer requires knowing 3 Traits on the gear that you want to make so start researching traits now.