Start off the game with a Faerie Charm, Sight Wards, and a mixture of potions. In more organized play or versus stealth champs, you can also buy a Vision Ward instead of a Faerie Charm.
Sightstone is like the "new meta" for supports. With the removal of Heart of Gold, it fulfills the need for both health and wards. Other items for early game are Philosopher's Stone for regen to sustain your pokes (and to turn into Shurelya's Reverie later), plus Boots of Speed to help against skill shots and ganks.
Early game optional items:
These are some other items which help in lane phase. Chalice is a useful item to help with your mana consumption, and it turns into Mikael's Crucible later on. The magic resist also makes it nice to have in lane versus enemy laners that do magic damage. Kage's Lucky Pick turns into Shard of True Ice, Twin Shadows, Morellonomicon or Will of the Ancients. AP is not that beneficial for Sona's early game. Her Power Chord harass damage is dependent on magic penetration and levels. But if you want to build an item that Kage's upgrades into, you will need to pick one up sooner or later. If you are dominating your lane and/or you "accidentally" stole a kill, also consider Haunting Guise. Because of her Power Chord damage scaling on magic pen, Haunting Guise is pretty good early game on Sona, and the health makes her tankier too. But this is really a luxury item. It goes well with Sorc Shoes to dominate the enemy lane through the amount of burst your Hymn of Valor and double damage Power Chord does.
Ruby Sightstone is just the upgraded Sightstone. Ionian Boots of Lucidity are for the boost in cooldown reduction. If you need tankier boots, there's Ninja Tabi or Mercury's Treads. Shurelya's Reverie is built for the stats and active. The active is a useful all around supportive active for initiating/chasing, escaping, countering kiting, and helping out carries who don't have mobility skills. I prefer Aegis of the Legion as my other core item for its mixture of both armor and magic resist because Sona benefits from having the tanky stats to get off a good Crescendo.
With these items, you have a solid combo of health, cooldown, armor, magic resist, a defensive aura to compliment Aria of Perseverance's aura, plus a very useful active.
Highly Recommended Final Items:
I suggest these items as your last items.
Mikael's Crucible is a situational pick versus annoying crowd control teams. The active combines Heal and Cleanse. It's excellent against focus fire in mid-late game team fights, and to remove statuses when your carry gets initiated on. The active can save your carry while they're still going through "puberty" in mid game, especially carries who don't have blinks.
Zeke's Herald is a good source of cooldown reduction, but the aura was nerfed during the preseason patch. Regardless, I find it particularly effective on Sona because it'll double
her Hymn of Valor AD aura (!!!). In this particular build, it also provides more cooldown reduction (since I recommend Aegis of the Legion over Locket of the Iron Solari).
Lastly, Aegis of the Legion can be upgraded to Runic Bulwark for the additional magic resistance. This is useful against a fed AP champion, especially one with AoE damage, or versus teams with double AP.
Kage's Lucky Pick turns into Twin Shadows which is strong for initiating if you happen to spot someone who has overextended. There's also Shard of True Ice, which helps you support your initiator. I find Twin Shadows more generally useful/broad in use between the two because it also gives you movespeed for chasing and escaping.
Locket of the Iron Solari can be built as your tanky item instead of Aegis of the Legion if you'd like. You'd want to build Locket of the Iron Solari instead when the enemy team has no AP (double bruiser comp). In contrast, you'd definitely want Aegis of the Legion/ Runic Bulwark versus double AP. Every team benefits from having both items in team fights though. If your jungler is building Aegis of the Legion, you could buy Locket of the Iron Solari instead (and vice versa).
This is my primary recommendation for a support enchantment. The description for Enchantment: Captain is a little misleading. Your allies don't need to (literally) be running toward you. They can be facing you (even from the side) and they'll receive the buff as long as they're within 1100 units of you. I also recommend Enchantment: Distortion because Sona can make big plays with Flash and Crescendo. The generic recommendation is Enchantment: Homeguard to defend your base easier.
Early Game Lane Dominance:
Sona's harass scales very well early game with magic penetration because of Power Chord. So, if you're fed/ahead early game, the Sorcerer's Shoes and Haunting Guise item combo will make you nearly do true damage to the enemy carry/support who only have 30-45 magic res, you'll be able to dominate them with your poke damage, and you'll still gain flat HP from the Haunting Guise.
These are some other possible/situational items. These are some other possible/situational items. If you took Heal, you can build Eleisa's Miracle. Buying it too early might set you back (it's nearly the same price as a Ruby Sightstone). You don't want to buy it later than level 15 though because of the passive.
If you're fed/ahead during mid-late game, Athene's Unholy Grail is an alternate build option for your Chalice of Harmony. You get a cooldown reduction boost and a lot of mana sustain during team fights. It's very easy to get assists on Sona with her auto-target spells. Iceborn Gauntlet is a tank version of Shard of True Ice. It's incredibly expensive for a support, but works hilariously well on Sona because of her low cooldowns and quick, smooth autoattack. It's another option for you to consider if you're turning into Sona Moneybags-McDuck.
Tear of the Goddess is not supportive, and it has very little impact on your team compared to other items. For 150 gold more, you could buy a Kindlegem instead. If you're really having mana issues, get Chalice of Harmony which turns into the supportive Mikael's Crucible. And because of how the Morellonomicon passive is applied, it has low synergy on Sona. Your Hymn of Valor is autotarget, and your ult will apply it in an AoE, but that tends to be used on people who are above 40% health to initiate fights.
These items sound like something a support would build, buuut...
Banner of Command compliments a fast push team. Lulu is often combined with Caitlyn for a strong early lane push, but that relies entirely on their AoE damage with Glitterlance and Piltover Peacemaker. It'll be far past lane phase once you get the gold for Banner of Command. It has poor base stats for the cost, but the active makes a siege minion go Hulk mode. Outside of push comps, it can be powerful to have a promoted minion wailing away at bottom tower while you and the enemy team have a Mexican standoff at baron, but that's a very specific use.
Ohmwrecker is an "anti-turtling" item. If the enemy team has a fast wave clearer like Morgana or Anivia, minions die too quickly at towers. Ohmwrecker removes the threat of the tower and forces action on teams that are trying to turtle. Like Banner of Command, it also has one very specific use.
Use Sight Wards in addition to your Ruby Sightstone when you need to heavily ward up an area such as Baron. Vision Wards are powerful early game versus stealth characters and to clear enemy wards, but buying too many Vision Wards can set you behind. Later, buy an Oracle's Elixir before going after objectives to clear out enemy vision. And when you know you're going to group up and try for an objective, there's always Elixir of Brilliance and Elixir of Fortitude to boost your stats.