I've been playing Sona ever since season 6 and I've been toying with different kinds of items on her. Sona has a lot of burst and is one of the best supports for she has heal, slow and stun. Since it's difficult to solo queue as support, I usually go support carry which is in handy- especially when your adr sucks.

Recently I've been starting with Ancient Coin because I get gold faster and the minions drop mana coins (or whatever it is). Only go for moderate pokes when taking Ancient Coin in the start because of your mana regen. Just stay alive in lane and get as much as gold as you can while poking your enemies.

Spelltheig's Edge is also good since it increases your mana regen and you get AP. It's a better choice if you poke and heal a lot in lane.

Don't fully build the first item, only do it if you earn 500 gold with that item. Start with Athene's Holy Grail for your shields, mana regen and a bit of AP or go ahead with tear, don't fully build tear yet until you're near the mana goal thing so you can upgrade it. Buy boots if you have a spare 300 gold. Buy Lich Bane because it scales well with your passive and skill 1.

Last item could be either Luden's Echo, Rabadon's Deathcap or Redemtion. It could also be morellonomicon if they have tanky champions. I prefer Luden's Echo since it also gives movement speed but Rabadon is also because of the 40% AP. Buy Redemtion if your ADR is actually good and would actually be usefull when you heal them.

In team fights:
If they have a healer support, go for the support first. If not, go for their ADR. But don't forget to heal your teammates.