Hello people, my name is frost shinigami and i will show you how I play SUPPORT sona. This guide will focus on every aspect when it comes to supporting with sona, e.g laning, teamfighting, harassing and generally making your carry's life easier. ENJOY :)

PROS
great poke/harass
spammable heal
movement speed buff
great at teamfights with Crescendo
CONS
very squishy
heal is kinda weak
mana hungry when spamming

GOOD OPTIONS
Flash
flash works with sona as it works with every champion in LOL, as a support you will be spending your time with your carry or warding, supps should scrifise their lives for theit carry, but that doesn't mean you should feed, so always take flash, to provide yourself an escaping tool.
Exhaust
Great spell, can be used to either secure kills for your team, save a teammate, or your own attractive butt ;p
Heal
always a good options, you can either pick this if you are asked too or pick it in a passive lane e.g when your AD carry doesn't synergise well with sona
MEH...
Clairvoyance
This is a totaly personal opinion, some of you may find this spell awesome when it comes to supporting, but i just dont like it...
Ignite
not a bad pick, you can use it when starting a fight with the enemy carry to reduce his health and healing effects, but i dont use it all that much...
Clarity
this is a good spell when spamming A LOT, but i find exhaust and flash to be more important, besides you DO have your Chalice of Harmony
BAD OPTIONS
Revive
this spell is ABSOLUTELY USELESS, it comes in handy only when DYING and its cooldown is like 500 secs!
Smite
do you play jungle? NO (unless you are phreak)

As sona you are veeeeery squishy, these runes and masteries aim to make you a little bit durable so you wont be dying or recalling that much... also as a supp you wont be getting many kills, so the gold quints could be very usefull, you can also take movement speed or armor, which are good choices too.

Passive: Power Chord
this is your bread and butter when it comes to harassing, once every 3 abilities sona's next basic attack will have a bonus effect depending on the third ability sona casted, when the third ability cast is your Q you will do bonus damage, when it's your W you lower the enemy's armor and magic resist and when it's your E it will slow the enemy.
Q Hymn of Valor
when casting this sona attacks the enemy with music, dealing magic damage, this ability automatically targets enemy champs, good for harassing.
W Aria of Perseverance
your heal, when casted sona plays some sort of beautiful music, healing the most wounded ally champion, this heal is kinda weak but spammable.
E Song of Celerity
This is your movement speed buff, when casted sona plays music (surprise surprise) and gives bonus movement speed to all your allies.
R Crescendo
this ult is nuke skillshot that fires such an intence tune that makes enemy dance stunning them, this ability shoud ne saved for teamfights, but can be used for securing a kill in lane too.
*NOTE
after casting one of your abilities sona will get surrounded by an aura, each aura passively buffs all allied champs around her, each aura has its own colour: blue, green and purple each one representing one of her abilities
blue aura: gives all your allies bonus AD, can either help your carry farm or help him in fights
green aura: allies around sona enjoy bonus armor and magick resist
purple aura: allies enjoy bonus movemend speed.

BOOTS
when choosing boots for sona you can either pick Ionian Boots of Lucidity or Mercury's Treads ionian could help you spam your abilities more often, allowing you to support you carry even better, while mercury's will make you extra durable in lane.
OFFENSIVE ITEMS
here you can pick Will of the Ancients athen's unholy grail or a Morellonomicon personally i think that will of the ancients and unholy grail are better options, since will of the ancients also support allies and athen's build from chalice.
DEFENSIVE ITEMS
here you can go with Mikael's Blessing Locket of the Iron Solari or an Aegis of the Legion and Frozen Heart all of these items are great so pick 2 of them for your build.

I hope you liked my guide and helped you understand how to play sona better. make sure to like, and comment, all feedback and comments are happily accepted ;)