Gear:
Tl;dr - 5 pc spell power cure, 5 pc worm cult or sanctuary or twilight rememedy, maelstrom lighting staff and restoration staves or master restoration staff or 1 pc molag kena/troll king/sentinal of rkugamz

The absolutely essential gear set you need is Spell Power Cure, obtained from White Gold Tower. This thing is by far the best healing set in the game, adding so much dps to the group. The other sets are a bit more flexible. Depending on the situation and/or preference, you can run Worm Cult (when the dps are both magic and you can tell they have sustain issues) found in Vaults of Madness (this is my preference since it also effects me, and most people these days are running mag builds) or Sanctuary (when you have squishy teammates or you just like god-mode amounts of healing) found in Banished Cells. Twilight Remedy is a great set if your team is good at activating your synergies, which you have no less than 3 of, 4 with your occasionally ultimate, but most pugs are not good at this so only run this with a trusted group. Found in Maw of Lorkhaj. The last piece should be maelstrom restoration and lightning staves, but if you can't beat VMA then use the master version of the staves. If you can't beat VDSA, then use some random powered staves. Alternatively, if you haven't gotten either of those two 1 piece staves, you can use a set piece staff from one of the two 5 pc and use 1 pc Molag Kena, Troll King, or Sentinel of Rkugamz to get the full effect your gear.

Rotation
The main idea of this build is to heal at all times via passive heal over times (HoT) and provide buffs, debuffs, and resources to your team. Your back bar is you main set up bar. Put down extended ritual, hit rapid regen twice (it only hits 3 people, so for a group of 4 you need to hit it twice), channeled focus, put combat prayer on everyone (ideally one will hit everyone but if not chase your team down, this skill gives great buffs), bar swap, radiant aura, elemental blockade, blazing shards on the tank, 3 to 6 orbs through the group (to make sure everyone has access to one and to make sure if they miss one another is nearby to get), heavy attack twice, bar swap, combat prayer, bar swap, blockade, shards, heavy attack once, bar swap, repeat. Usually you won't need to use breath of life, but if you're running with especially squishy people or someone makes a mistake be sure to hit that oh sh** button. Use warhorn usually for the ult, its amazing, but if the damage is fine but people are getting hit hard, use Nova to reduce damage taken by 30% and give huge synergy damage with an aoe stun.

This build gives magicka back in two ways, stamina back, does a bit of damage, gives 18% boost to damage, gives almost 250 spell and weapon damage, gives a bit of resistance, and could potentially give extra crit damage or reduce magicka costs of your entire group, with an ultimate that improves damage by a ton or deals a ton of damage and reduces damage taken for your whole group. Talk about buffs.