Hello! This is GnawAndSky, and this is also my first guide, so sorry if it isn't the best guide out there. This is purely how I prefer to build Sona, and I believe it works well.

I also think that Sona is a good support, despite people telling me that she isn't. I don't know very much about runes at the moment, so I'll update it later when I learn more about them.

For Sona masteries, I go for plain support masteries, it works fine.

4 in offence, because Cooldown Reduction.

5 in defense, because it keeps Sona on the field longer.

21 in utility, because it gives Sona biscuits, Money, Mana, etc.

With Sona, you need mostly Mana, and a little bit of Ability Power, so she heals more with her W, and does more damage with her Q and R.

Starting off the game you should get Ancient Coin, because its an awesome support item.

I highly recommend getting Ardent Censer, because it supplies Sona with great Mana Regen, and a little AP, as well as its very useful passive.
Another useful item is Athene's Unholy Grail. It gives us Mana Regen and a bit AP, with some Magic Resist, which is also useful.
Depending how the game is going, you should either buy Archangel's Staff, then boots, or the other way around.
Next, the Abyssal Scepter. This is useful because AP. AP is useful for Sona because it greatly increases the health her W gives, and increases damage on her Q. The Abyssal Scepter's Aura is very useful as well in teamfights.

I get Aria of Perseverance first, to keep the ADC on the field longer, and to keep my health up.
I get Hymn of Valor second, to do damage to minions and enemy champions.

I get Song of Celerity third, to avoid ganks, and the others are more important.

Max Hymn of Valor first to do as much damage as possible.