This is my Sona build. It started as a Build but the First commenter made me decide to write a short guide so people can understand my choices.

Pro's
-Clairvoyance makes you irreplaceable if done right
-Healing is Amazing
-Ulti stuns and is AOE so can be the difference between life and death in a Teamfight
-Passive is just plain ******ic if you're good at build powercord up and using the correct spell for the moment.
Con's
-MS Skill isn't very useful aside from it's Passive's effect for the slow.
-This specific build doesn't do massive damage.
- This last one isn't really a Con but a LOT of people complain about it. Sona won't get all that many kills she will get tons of assists and low deaths but not many kills. Average game I score twice the amount of kill to death but 10x the assists (i.e. 4/2/20)

Marks- Come on this should be self explanatory not too much else for casters to choose from.

Seals- On Sona MP5 is probably the best thing because early game Sona is very very Mana hungry and without Clarity you WILL need these if you plan on staying in lane for awhile.

Glyphs- I went with more MP5 because I hate recalling for mana early game because it really cuts into your XP/Gold farming. CDR Glyphs are unneeded because you have 39% already from masteries and Items. The only other Glyphs that would be useful are AP/AP/lvl or MR.

Quints- these help a lot early game for extra Heal power and they're just plain good.

Always start with Meki Pendant Sona is very very mana hungry, I also take an HP Pot and MP Pot. I then go straight into CDR Items so that I can always have heal and speed up when needed Cool Downs are your worst enemy on Sona.
Archangels' pretty much takes care of your Mana Regen problems. The rest is AP AP AP! you want your heals to be healing for 400-600.

If playing a 3v3 the game will usually end a little after you get Abyssal Scepter so the last 2 Items are for 5v5's. SONA DOES 3V3's very well her Heal only targets the most wounded so in a 3v3 you have much more control over it.

Rylai's is a typically good caster weapon but on Sona it kind of sucks, her only damage spells are her Q which hits 2 people so Rylai's passive only does 15% rather than 35% and her Ulti stuns them so by the time it wears off so does Rylai's passive, these things make Rylais really bad since you get a really expensive item with sub par stats and a passive that hardly does anything.

Lich Bane is Amazing and if you feel like it will serve you better than Rabadons, Zhonya's or Abyssal go a head and take it.

If the opposing team is being really hard on you with their physicals go with a Frozen heart over Morello's you will lose AP and Magic resist but gain Armor and Mana and an amazing passive really up to you on the matter of frozen heart depending on the game. If getting Frozen build ToG first.

Mejai's is an amazing item for it's cost but I don't like it because Sona is support which means she won't be getting many of the kills so it will take a long time to build stacks on it and if you die you just want to shoot yourself.

If their team is Overly CC get Quicksilver sash over Abyssals.

NEVER LANE ALONE always go with a partner. I'm not only talking about early game Sona should never ever ever be alone because she's very squishy even with Heals MS and Zhonya's always be with someone. Something amazing to have while playing as Sona is a Speech system such as Ventrilo, Skype or Teamspeak so that you can better coordinate, I have been in many a game where I was running to try and save a teammate with heal but they were running the wrong way and I couldn't type to them fast enough to try and tell them to turn around so I could catch up to them and help them either finish off their chaser OR help them escape through my MS buff.

Sona can be built for Damage but I find her best when she focuses on healing, by focusing on healing she actually does more damage because she stays alive longer and she lets her teammates do more damage because she will make them stay longer as well. That's why I go with getting Aria of Perseverance to 5 first, it's by far the most useful.

Clairvoyance is a MUST have on any good team. It gives your team a way to check baron and dragon without wards (wards are still good to grab though) it shows you who's going in what lane at the start so you can best face them. It can be used for checking bushes for a lane member who's wary about it and doesn't have a good spell to do it for them. Clairvoyance is Support and so is Sona so of course they go well together.
Clarity is ok because early game it's amazing but it dies late game.
I prefer Flash because it can be a very good run skill since Sona already has a MS Buff and can also be for getting that last hit on that fleeing opponent.
Ghost has similar reasoning to Flash.
Cleanse. This is the skill I would take if you didn't like Flash. Cleanse is just so useful in both running and Teamfights, if you get CCed in a teamfight Cleanse can be your savior by allowing you to get away / heal.

HAHAHAHAHAHA don't even go in their for the buffs since Sona does not need them. Blue buff is better for your DPS Casters Red buff is for your Melee If you're high level and you're going through the Jungle to get between lanes use your Q as you move and perhaps auto attack once just to get gold other than that stay away from the buffs and jungle creeps.