Of the four classes the Templar is the only class that can heal without using a restoration staff, this makes it a particularly potent support class. There are one main archetypes that I’m going to discuss, both focus primarily on supporting a party rather than maximizing independent DPS for solo play.
The Ranged Support
The ranged weapon can either be a bow user or a staff, but in either case his role is to provide crowd control and support from a safe distance. Since you shouldn’t be taking too many hits feel free to use light armor and dump all your points into magicka or stamina. You’re going to want to be either a Bosmer (for bow) or a Breton (for caster) as they provide the most useful Elder scrolls gold for a ranged support Templar.
As a ranged support, you will have a perfect view of the entire battle, allowing you to heal allies wherever needed and assist in focusing down priority targets. Use Rushed Ceremony to keep everyone’s health up, and Sun Fire or Scatter Shot to keep enemies snared. For your ultimate Nova is probably the best option but Rite of Passage is still good.
This is just a guide. Naturally you will want to adjust your build according to your specific needs depending on the dungeon, your party’s composition, as well as your own personal play style. After all there are many ways to build a support character. The important thing is that you assist your teammates, however you choose to actually do it.