Ok, in this guide for Sona, we are going to go support/sustain. The most important part about playing with this guide is to be patient, like with all supports, because come mid to late game you will be golden.

First is your Rune page. It's pretty standard, AP for your Quins, Magic Resist for your Glyphs, Armor for your Seals, and lastly, Armor for your Marks. If you have this specific rune set, you should have 15 AP, 12 Magic Resist, and a total of 21 armor. It should look like this:

The mastery page is key in this. Tentatively you should have 22 points in Defense and 8 points in Utility. Your mastery page should look like this:

For your summoner spells we go full support. Clarity and Heal.
Clarity
Heal

These 6 items are the end game build or if you've managed to get fed enough to get them quickly. I myself have never made it past the 4th item on this list simply because in mid to late game when team fights become more frequent, the team I'm on always wins due to us either taking the nexus or them surrendering. The items are as follows: Merc. Treads w/Alacrity bonus(or Homeguard/Captain totally up to you), Athene's Unholy Grail, Frozen Heart, Runic Bulwark, Rabadon's Deathcap, and Zhonya's Hourglass. The reason behind these specific items is to help you stay in the fight longer and continuously heal without fear of mana depletion or cooldown's. The grail and heart max your cooldown reduction at 40%. The primary purpose of the heart is to slow down your opponents so the grail can do its job, increase mana regen as well as restore 12% of it with kills or assists. Next is the bulwark. The added armor and resist just doesn't help you, it helps the team along with hp regen, combo that with your perseverance aura, all your allies will have bonus armor, resist and health regen making the decision on who to take out first more troublesome because the team is constantly regaining health. The two heavy AP items, rabadon and zhonya's are there for just that, to increase your heal and attack abilities. The deathcap gives you 120 plus an extra 30% of whatever your max AP is. Throw in zhonya's for an extra 120 plus 50 armor, tack on zhonya's active to make you invulnerable, allows you to stay in the fight that much longer so your aura buffs stay with the team. Lastly is the Merc. Treads w/Alacrity bonus. This is a personal preference, because if you don't use boots, you can easily swap this out for more AP/HP regen/MP regen, or whatever suits your fancy.
Athene's Unholy Grail
Frozen Heart
Runic Bulwark
Rabadon's Deathcap
Zhonya's Hourglass
Mercury's Treads
Alacrity Bonus

As far as your skills/abilities go, it's best to start with your heal first, then grab your speed, followed up by your attack. After you obtain all 3, focus on your heal till max(grabbing your Ult in when you can) then hit your attack and speed ups. This is how I usually roll with it:

I normally start with two faerie charms a ward and a potion. The charms are good to keep up your mana regen so you can farm as much gold as possible before heading back. The ward is a standard for any support, knowing when the enemy is coming is always good. The heal potion.......honestly, not sure why its there lolz.
Faerie Charmx2
Sight Ward
Health Potion

Side Note: This is all you will need to start with. Once the game gets going and you start getting into your Unholy Grail and Frozen Heart, you will start to see the difficulty your opposing team is having with you.

Now this is what I personally do while playing as . After you obtain your Runic Bulwark, you're pretty much unstoppable. Your ability to constantly stay out in the battlefield and heal your party will be OP as all hell. Since doing this build with I have yet to lose a game. The best end game stats I've obtained was 8/5/21. This build has been tested and I use it constantly. So feel free to test it for yourselves and leave feedback on how it works for you. Again, this build works amazingly for me and I enjoy it, but if your play style is different from mine in that you want to be more aggressive, this may not be the best choice for you. This build is pure support and is meant to be played as such.