This is exactly this Alcast's build:

https://alcasthq.com/eso-necromancer-healer-build-pve/

Use a Hollowfang resto staff until you farm the Master one in Dragonstar Arena.

Other nice monster set which is MUCH harder to obtain is Symphony of Blades.

Other 5-piece I like os Roaring Opportunist which could be paired with Spell Power Cure for more damage if your group has no magicka sustain issues.

Beginner 5-pieces which can be obtained in the base game include Kragenac's, Sanctuary, Worm's Rainment.

Always keep your AoE dots on as many targets as possible (Graveyard applies major breach and Wall of Elements off-balance) and keep Elemental Drain on your priority targets.

Always keep up Summoner's Armor and Spirit Guardian.

Orb and Illustrous Healing will help with your group's resources and Combat Prayer (important!) will buff their damage.

Radiating is a group heal and Resistant Flesh is an emergency button (and there for passives).

Your ult is warhorn, always use it. On emergencies you can use the mass ress ult or just trade it for the mage's guild or for the Colossus (greatest dps ult in the game)