This guide will lead you to being a Sona that never needs to go back for mana, can cast fast enough to get lots of heals off in a fight, and can harass enough to nail down assists.

First off. First guide ever, appreciate tips, I know its not flawless.
Second. Props to Chrispykreem. Your build led me to loving to play Sona and eventually to making this build. That build I used for a while, but I've build my own that I've found keeps me in the fray and the most helpful to my team both as a caster harass and amazing support.
I realize there is some redundancy in stats at 18, but I find hitting those caps ASAP makes you powerful support mid-game when it greatly helps set your team up for an awesome finish.
You should be a rolling support mini-gun by mid game and always with your group. When I play Sona, my goal is three things:
- Keep players alive when they may have died
- Keep players fighting when they may have returned to base
- Keep group sticking together and dominating team fights

Runes are:
9 x Greater Mark Insight = Spell Pen
9 x Greater Seal Clarity = Mana Regen
9 x Greater Glyph Focus = CD Reduc

3 x Quint Potency = AP

This is the core of my build. Mana regen x CD reduc x AP
Keep the spells going constantly, keep mana up, and up the power.

Like the rest of my build:
- mana regen
- cd reduc
- keep you in the fight
- support team

Not too picky about this but I get clarity point and teleport point. Those two abilities I pick every time.

Teleport: Get the mastery. Watch your CD on it. When its within 10-15 sec or free, and you got gold to spend, USE IT, and jump back to your team fight. If there is no one out there, take your time and walk and save it for later. But use teleport every time you THINK you need it.

Clarity: Early game, use it to keep your lane mate pushing and harass theirs lots. Use in team fights to mana up allies. You will have enough mana regen by mid-late you won't need it yourself, but it keeps everyone going and topped off after that big push for a second one, when their team is going back or hopefully dead (because your team is all alive and killed them.)

I'm going to sum up items and play all right here. Make this guide no longer than I need to.

Pre-game:
Pick up Meki pendant and first point in Q. Don't go in bushes first. Don't wander out. Don't lane with a tank or solo mid (if you can, tanks should support their lane) Support a carry like Xin or Jarvan, or someone that would benefit from having some support.

Early game:
Harass. Harass your opponent. No one is scared of Sona early game. Use this. Do "swoops". Learn your range on Q well and when they get close to your minions, swoop in and hit them with Q. Don't use W much on rank 1, it doesn't heal worth its mana at this point. (Unless you need it.) Get rank 2 on it at lvl 3, then heal up. Clarity whenever you OoM and share it with your partner if they use mana. Always keep them topped off. Once you have 605g, quickly grab tear of the goddess and port back. Keep your partner topped off and try and push that turret. Don't be afraid to use R once you get it at 6, whether it be to nail down your teammates kill attempt, or to save them from your enemies. Having 1.5 seconds to get away WILL SAVE YOUR TEAMMATES. One quick R-W and your team mates are safe. One quick R-Q and your enemies are spooked.
By mid game should have Tear of Goddess, Boots of Speed, and Fiendish Codex.

Mid-game:
Your mana regen from tear and codex should be high enough that you should be doing something like this:
Q-W-Q-W-Q-Q-Q-W .....lots. Your lane's enemy tower should be down. You should have spent about 9.892 seconds in your base all game (just enough time to buy items and port out).
Stay behind your minions, but always with another player. Never ever ever ever be alone, unless its to run from one ally to another. Announce to your team whenever you are going to leave because they will get used to your awesome support. Finish off your Morello's Tome and boots, and then either pick up needless large rod for the deathcap (if you have got lots of towers and assists) or finish off Archangels staff.
If you need they HP get the belt and put together Rylai's to up your survivability.

End-game: Your CD and mana regen should make you a harass heal zoom machine gun turret.
Q when enemies get near, you should hit for 300 or more and with fast CD, a few hits to a Ryze or Kog is what makes them a quick snack in a team fight for any of your allies.
W to keep everyone topped off at full hp, (unless they don't want full like Olaf who does more damage with less hp)
E to make your allies and minions zoom to a turret, to chase an enemy champion, or to quickly jump to another lane.
and R. This one is the one that will make you go from a good Sona, to a WOW-HOLY-MADNESS-GG-SONA-YOU-ROCK. This is what I usually do in a teamfight to set up my R.

Stay near your team, but at the front, draw the enemy in (Sona is good at that *wink*) and once 3-4 of the enemy champions commit, hit as many as possible with R JUST ABOUT when they are about to unleash. Your team should jump in at this point (or be in the fray, thinking they are about to get owned.) Your stun and Q should hit a couple enough for two easy kills,, quickly heal frantically, use E to zoom any nearly dead out of there (sending them off with a W) and keep Qing them. Using your Ult on 4-5 of the enemies should set your team up for a four or maybe five kill, and then heal up your team and zoom them in to down a tower and maybe inhibitor. If people need mana and you lost two or more, W your team out and port back.

This is lots of my average games looks like.
But, you will not always have high scores. You may come out of a lot of game with a score like these:
2-1-4
0-0-5
4-4-10

Those don't look that great. But then when your Xin-Zhao has 14 kills that he got because you healed him, or perhaps just the fact that you saved Warwick from certain death numerous times because your heal and run speed got him away from four enemy champions chasing him down in the bush....
Basically, You will lead your team to a strong victory but most likely your glory will come from your team mates shouting praises to you, not from the numbers on the scoreboard.

The words behind those scores:
2-1-4 - "OMG Sona, thank you soooo much for those awesome saves!"
0-0-5 - "Sona, they never got ANY kills. That game was SO FAST!"
4-4-10 - "Sona, you are a goddess. I you props on half my kills."