This is NOT a build for beginners.
Pretty simple. If you are gonna die, then go find some other guide.
Magic resist Sucks, Armor Sucks, HP sucks, HP regen sucks, mana regen... it's OK if you can't afford AP Runes yet.

OH YEA and screw Armor/magic pen BS, you don't need it, it is a waste, a big fat waste. it is good for one only ONE thing! AP is good for THREE!!! It will boost you damage, heals and your run speed boost! screw everything else, AP and Skill is all you need!

YOU WILL NOT BE INSTANTLY GOOD BY USING THIS GUIDE.

This build works for me because I know were to go, how to move, how to groove. I am not afraid to run across the map to assist my team and run back to bot, whatever, because I will stay just out of your range 80% of the time, as desirable of a target I am for my low HP, I am annyoing as f*ck to try and catch. I know how to dodge, how to escape and use my combos effectively. Sona of course is a pretty easy toon to use effectively, but it is up to YOU to take it a step further then that with your OWN skills.

If don't dodge very well, or anticipate attacks very well, this guide is NOT for you. gtfo cause you will die. Your team will be very very mad and you will blame me because you thought that this guide was magical or something.

This is the order I build. Very simple, Rush the claim, it gives you more AP, and you can slow, this is useful in both aggressive and defensive situations.

Next, Asap, boots of mobility, alternate option is swiftness, but I have found mobility to be perfect for me in those fights when your on the edge, the are gaining, but you leave combat and burst out with a grain of HP left.. If you want to survive, you must move FAST, and position behind team mates, prepare to run always, if you can't save your guy, then leave em, staying to die with them will do no one good. Depending on my gold and how the first 5-10 min went, I will rush both these items about the same with a bias to Frost queens.

Next, lichbanelichbanelichbane perfect compliment to sona damage, heals, and mana pool, just about any other sona guide will agree on that.

Next, I have found that building into morellonomiconakjshdikajhsdhuaisuhd jesus that name is hella long, anyway, it's awesome for sustain in your run away OP damage catch me if you can combo, it keeps you going and adds more damage.

Neeext.... if you make it this far the rest of the build is really a matter of what you like. I like Void staff and rabbadabadabdaba... cause bursting down those squishies for your team is really handy.In the right setting you can get very close to one shotting, which is great to give your team mates free kills.

BUT, depending on the situation, you can totally carry your team back up by building into Locket of iron solari and having that extra shield.

And remember bronzie! you wanna move up to silver buy that ******* elixer, quit forgetting about it in your end game.

You might say, but duuude what about wards bro? What about defense bro?

No matter how many times I try it, my win rate plummets when I do. why? Because while yes, site is handy, it takes away from my AP. For how I play in this style, the free wards are plenty. And between all your team mates it should not be up to you to ward for everyone.

Ability power for sona means.... More burst damage, Greater heals, Faster run buff, and higher damage ult. Your best defense right there is in your abilities. You can burst, you can escape, you can heal, and you can stun. Not to mention your first item is VERY defensive, because it slows them down. I save champs all the time with it as well as myself.

What more do you need?
Oh yea, learn to aim your ult with quickie cast just because.
This guide is adaptable to all play styles, aggressive, passive, defensive. It is simply up to you to use your skills and move fast and be in the right places.

Do NOT get MAD if you die using this guide, this is a do or die kind of thing entirely dependent on how well YOU the player can play. Dodging skill shots is HUGE if you can't dodge them you are screwed. If you pay good attention to it you can predict when someone will use one on you or simply react quickly enough to do so.

If you cannot do these things, you should learn how to, if you cannot do that, you should play a different toon, or find one of those wussy guides that say build into Armor and MR.

I'm not gonna boast some cloud nine BS or how pro I am or any of that. ooooohh platinum/diamond/smurph you so gooood. woah bro.
Here is what I know as a casual player who doesn't have the time to grind all day.
When I lose, my team fed a lot and consequently so did I, sometimes it's my own fault, but we all know how solo que can be. When I win, it's not because I got carried, in fact, it's probably because I was the one who carried. We are talking about getting double digit kills and assists, or if your team is good, having assists for 80-90% of the kills made. And even when I lose I still often get an S score, and I have even gotten S+ on some losses, maybe that is shocking, maybe it's not, I feel like I do pretty well. I think you will do pretty well too if you can handle it.

So on that note,
Good luck, have fun, GG WP.