Sona'sHymn of Valor 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 granting additional AD and AP as well as 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.
Stay with the team. 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's auras are most effective when the team groups up. The gold value of Hymn of Valor is 144g of AD and 88g of AP per rank, maxing at 720g/440g. Aria of Perseverance is worth 240g at rank 1, up to 400g at max rank. Song of Celerity is worth 52g per rank, maxing at 260g. (These values do not include bonuses from AP scaling of the auras.)
Sona can twist her auras, having the effects of two auras overlap. The previous aura persists 1 second after she switches. At 40% cooldown Sona can switch auras every 4.2 seconds, giving two auras at once nearly 25% of the time. Constant switching is very mana intensive.
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.
Other 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
As midlaner, on the other hand, AP items with side utility should be prioritized, while getting CDR as close to 40% as possible. Athene's Unholy Grail is a must, while Rylai's Crystal Scepter andLiandry's Torment are good to have items to make you both slightly more durable, and increase power of your poke. Lich Bane, another useful item, is however ignored by some in order to achieve higher AP and therefore stronger teamfight impact, sacrifcing personal power for stronger supportive power.
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.