How to build Sona

There's a number of things that go into building a champion when you're looking at itemization. The Runes and Masteries are also important but they're going to typically be decided before the game. I'm assuming that most people have a set in stone rune page for supports and a set in stone mastery page for their specific support. If you have so many rune pages that you can choose between 2-3 for Sona then you probably don't need this ;)

So let's say you get in game. Here's what I'd suggest buying to start with

Take 1 point in the mastery that gives you an extra 25 gold and buy Crystalline Flask, Faerie Charm, Ward

Your goal is to stay in late for as long as you possibly can. You're probably going to harass a lot so you can delay dropping the ward until about the 3 minute mark when a jungle gank becomes possible (drop it earlier if you're pushing and/or they have a jungler likely to gank at lvl 2). To accomplish this, you need to aggressively control your own bush.

If you do all this, you can stay in lane for ~6 minutes. At that point, you'll have accumulated 600-900 gold (depending on when you backed, how much CS you accidentally stole with your Q and whether you have gold quints). Your wards have expired and your carry probably has a fair amount of gold to spend. You'll have more than this if you managed to bump off one of their laners.

At this point, you have four items that you should strongly consider buying.
Philo Stone (520): This is the typical support build. It will work well and adds roughly 50% to your gold gain. It helps your lane sustain and will eventually build into a "free" item that doesn't take a slot (not to be underestimated as a support since you'll have slots for wards taken up as well).

Tear of the Goddess (520): I like this when I feel like I'm going to be needed to add to the DPS total or if we're really bullying our laning opponent. Start building those stacks now so that when you buy the Archangel's Staff,  your getting fully value for it and can move quickly to the Seraphim's Staff:

Boots (350): I generally save these for my second back, but if you're playing an opponent who has a lot of skill shots or you sense that you're going to start roaming soon (because, for example,  you took their tower down already) then these are a good buy.

Sightstone (700): You always want to pick up wards but, on the off chance that you can afford this, it's generally the right purchase. Don't do what I did last night and back with 520 and buy the philo stone only to have no wards for the lane when you return. This item makes sure that you'll never make that mistake.

Once you've purchased those items, the general build to follow is this:

Ruby Sightstone (600)
Eleisa's Miracle (400)
Archangel's Staff: If you bought Tear
Merc Treads:
Aegis
Sheen: Whether Sheen grows into Lich Bane or Iceborn Gauntlet depends on your role. If you find that you're in the middle of team fights (perhaps you have the only decent initiation) then Iceborn makes some sense. Otherwise, Lich Bane is the more beneficial item.

All told, the build above costs 11450 (8750 if you didn't go Archangel's) which you should have in any reasonably lengthy game. It also leaves one - two slots open for other stuff. People seem to really like Sherelya's on Sona, I don't particularly. Especially with Lich Bane, I don't want my CDR to be sky high. If I spam abilities faster than 1 per 2 seconds then I actually reduce DPS on Lich Bane procs. Sometimes you need tankiness and something like Warmog's can help. Sometimes you need extra DPS so Liandry's, Blackfire Torth or even Rabaddon's could be in order. Rylai's and Twin Shadows can give you a little bit of both while Zonya's Hourglass can make you a great piece of bait. The last slot will only really come into play super late game so you want to make it something that scales well with you.