This build is specifically for someone playing Sona as the support. Sona is a brilliant support with reasonable damage, AoE ulte, a clever heal and extra team movement speed. What else could you possibly want from her?
I personally favor Sona over most other supports, as she has so many things she can do to assist the team.
In the guide I will give tips and help you to play Sona.

Greater Quintessense of Avarice: I chose these as when playing support champions you won't be getting much farm at all throughout the whole game. That means, that your going to need some kind of gold income from somewhere else. The answer is, GP/5(or GP/10) items or runes. So having 3 of these runes will help to bring in some income.

Greater Mark of Resilience: These runes are needed to help you become tanky. As the support, you need to stay alive for as long as possible so that you can keep healing up your teammates and providing that tiny bit of damage.

Greater Seal of Resilience: Again, the same as the Greater Mark of Resilience. They will help you to be able to take more damage.

Greater Glyph of Shielding: Once again, helping you to become more tanky.

I put 9 points into the Defense mastery tree to make you a more tanky support. By putting these points here you will gain more armour, magic resist and health.

The rest of the points I put into the Utility tree. These points are more specifically aimed at support champions.
Expanded Mind: 3 points into here will give you more mana, meaning that in early game you can use those few extra abilities with that tiny bit more mana you have.
Meditation: This gives mana regen, a must have for supports so that you can keep spamming ablities.
Greed: This will provide you with more gold over time, helping you get those items and wards quicker.
Intelligence: Cooldown reduction is also a must for supports as it means that you get those abilities back a tiny bit faster, which could result in that last second heal to a teammate.

This item build gives you a lot of tankyness. I find this very useful when playing Sona as it means I can take a lot of damage. So it doesn't matter too much if you get engaged upon, as you simply just keep healing, and keeping alive.
I start off with the GP/5 items as these are necessary to get some income. I then build boots, as otherwise your going to move way too slowly, but Mercury Treads also gives you magic resists and tenacity. I then build into Soul Shroud as this will give you a lot of health and also has quite a nice passive. Shurelya's Reverie is next. This is a must have. Despite is giving a decent amount of health and a good passive, it's active is very important. If you really you are losing a teamfight, use it and it will help you get away. Or, if your being chased down, it's very useful to make that gap, or even close the gap when your chasing. Abyssal Scepter will help to give you a bit more damage, and also your heals will do more, but it also gives a nice magic resist buff. And finally, Randuins Omen. This gives a lot of good stats, but make sure your tank knows th your getting it and they don't stack.

I max the W first, as this is your heal. This is very important in keeping sustained and keeping your AD carry alive.
I then max the Q. This will help to deal a bit of damage, but it's boost to other members helps a lot when in a fight.
Max the E last, as it's not quite important, and Shurelya's Reverie should have been build by then so you can use that instead for extra movement speed.
Obviously, ALWAYS level up your R at levels 6, 11 and 16.

Sorry for the people that don't use QWER for spells, but most people do.

Exhaust is brilliant to take as it means you can slow down the enemy AD carry that should be in your lane. It can also be used to slow enemies down when your getting ganked, but you want to try and save it for when your in a teamfight.
I also take Flash, as Sona isn't the best at getting away from situations, from this is good for getting away.

PROS:
-Good at sustaining in lane with the AD carry
-Reasonable damage output
-AoE Stun with Ulte
-Movement speed buff
-AD and AP boosts
-Tanky
-Short Cooldowns

Cons:
-very slow movement speed, unless you have Shurelya's
-can be hard to get into the correct position
-very squishy early game

NEVER, EVER even attempt to jungle Sona. I have tried and failed hard.
It's a 100% no.

Thank you for taking time to read my guide.
I hope that it will help you in the future when you play support Sona. She's is very fun to play, and if played well enough you always get "TY's" or "GJ's" during the game which boosts you even more!
Have fun playing Sona!