Hymn of ValorSona is very strong if leveled up quickly. However, because of her lack of escape mechanisms or survivability skills, she should still be played as a support champion.
Leveling first is great for aggressive bot lanes due to Hymn of Valor's passive aura providing decent harass during the laning phase.
In a passive support lane, encourage your carry to trade blows with the opponent, and do the same with Hymn of Valor and Power Chord. Afterwards, heal yourself and your partner with Aria of Perseverance. This will wear down the enemy and force them out of lane in less time and mana than just poking with Hymn of Valor.
Use Aria of Perseverance when an ally is being focused in order to take advantage of the aura's shield.
Stay with the team. Sona's auras are most effective when the team groups up. Sona makes an excellent healer, Crescendo is best used when the whole team can take advantage of it, and Song of Celerity lets the entire team kite. Without a teammate around, Sona is little more than an easy gank.
Sona can twist her auras, having the effects of two auras overlap. When a new aura is cast the previous aura effect stays up until its duration expires.
Early game, be wary of constant spell rotation, as it will quickly drain her mana. However, this can also be a large problem mid to late game without proper itemization. Chalice of Harmony is a valuable mana regeneration item for Sona.
Crescendo coupled with allies' channeled abilities can devastate the opposing team, as while stunned, they can't interrupt the channel.
Being a support, Sona should focus on supporting her AD carry early game by warding and keeping him/her safe. If your carry can get last hits, you'll want to leave them for him/her. You can offset your lack of farm by buying Ancient Coin or Spellthief's Edge for more sustain, better healing, and Gp10.
If you go Sona midlane, you usually need to keep your wards up, as chance to escape ganks, when not warded, is limited, and basically impossible if your flash is down. Getting fast Sheen is recommended, as well as getting Chalice of Harmony right afterwards. You also want to rotate your abilities, in order to have 2 stacks of your passive always ready to punish any overextending champion with Hymn of Valor + Sheen + Power Chord.
A valuable build for Sona is stacking auras, as all it takes is Sona's presence for her to be useful and most of the aura items you should be getting are relatively cheap.
Another offensive Aura is Abyssal Scepter. Its debuff over enemies increases the magic damage output of your whole team.
With her Song of Celerity and a Talisman of Ascension, Sona can make it nearly impossible for the enemy to catch a dying teammate or escape from her team during a chase. These two things also allow her team to traverse great distances very quickly, such as quickly running to a tower/inhibitor after an ace to take it down before the enemy team revives or moving to a friendly tower/inhib to prevent it from being taken.
All of Sona's skills (including her passive) scale with AP, but support Sona should prioritize cooldowns over AP. So to maximize the utility of her skills, passive and AoE stun of Crescendo
Sona is extremely valuable in group pushes, but sustained aura rotation can deplete her mana pool quickly if not equipped properly, consider mana regeneration glyphs per level to make sure she never runs out of mana.
Because Sona has generally low cooldowns, low mana costs, and needs to constantly switch abilities, she charges up Seraph's Embrace faster than almost any other champion. This also helps her, as it provides one of the highest mana regens in the game, allows her to stay in long team pushes for much longer, and gives her a small shield for incoming burst.
Lich Bane can add a significant amount of damage to her autoattack. Combined with Power Chord it can provide decent burst damage.
As an alternative to Lich Bane, a more supportive Sona player might want to get Iceborn Gauntlet. Because Sona's only non-ultimate Crowd Control is her Song of CelerityPower Chord, this can limit her choice of power chord when she needs to keep an enemy in place. Iceborn Gauntlet solves this issue, because its passive makes every Power Chord have a slowing effect. This means she can use it in combination with Aria of Perseverance to stop an enemy's burst damage and give her team's carry time to escape, or for higher damage with Hymn of Valor.
Sona's Aria of Perseverance can shield multiple allies at once. Because of this, Ardent Censer can be a very good item on her, providing cooldown reduction, mana regeneration and ability power, alongside an attack speed buff on her W. It would depend on her team's composition. But with multiple auto-attack reliant champions on her team, it could be a very strong item.