Hey there, still working on this build, so feel free to add (constructive!) criticism as I tweak and perfect. Also, this is my first build, but you could say support characters are my *ahem* forte.
I absolutely do NOT go anywhere alone; maybe defend a turret until a laning partner rezzes but, even then, hug that friggin' turret like Annie hugs Tibbers. Sona is SUPPORT, and has a big target on her head, especially if there's nothing around for her to support.
Clarity is a great choice to keep going and going in your lane and get to Level 6 Go-Go-Gadget Dance Dance Revolution a bit faster. Remember too that you don't want to just wait until you're getting low, keep an eye on your laning partner's mana as well! Teamwork, people! With this, I've never used mana potions as long as I started off with Meki Pendant.
Flash is my O.S. button. First tried out Ghost with her, but if you get the shoes, reverie, the speed boost mastery, and combine with her E, once you Flash out you should be safe. This is just a personal choice though; I barely used Ghost, and almost never past level 9-10 and it felt like a waste of a summoner spell. Besides, she should be in the back of the pack most of the time anyways.
Tear of the Goddess: I start out with Meki Pendant and a couple health potions just to last until I get my heal spell at level two. Later, when going back to the store, I usually finish the Tear and hopefully get started on the boots. However, I don't prioritize building an Archangel's. I just like the mp5 and, since I spam spells, my mana pool grows waaay fast.
Mercury's Treads: If I have to explain this, you shouldn't play Sona. She WILL be targeted for CC's first in group fights, you need to get out of there ASAP. 1/3 less panicking time.
Innervating Locket and Soul Shroud: General fun-time aura help.
Shurelya's Reverie: Even more group speed buff? Yes please! If nothing else, I hope this convinces you not to waste a Summoner Spell slot on Ghost. It even has a musical term in its name! Totally meant for Sona.
If you have the time/funds, I would suggest:
Archangel's Staff (for you completionists, or if you want to go more agressive and want the AP, but I don't go that way with Sona)
And don't forget to grab the occasional Elixir of Brilliance!