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You would want to max Hymn of Valor first since this is Sonas only damaging spell except her ultimate. But what you also want is the E spell aswell because, as this happens to me often that enemies get away with too low hp, and with E you become faster for 1.5 sec and can catch up and get that last hit with Q. W is important to survive in the lane if you are going mid for example, always stand behind minions and harass enemy champions whenever they get to close to harass you. Now you use W and you regain your lost hp and they keep losing theirs. Now when you are in lvl 6 and you got your ultimate, the enemy champion should have low hp and could easily be killed with one push on R and then Q. Stack your spells until you have 2 stacks on your passive, now dont attack minions or you will lose this VERY important damage and you will need it if you want to kill the champion. When he takes a step too close to you, you use Q and ultimate. If target is not dead keep running after him until you can throw Q again and BAM, you got a kill! Rest is up to your skills.
To explain this simply, Sona is awesome with these spells because with Clarity she can stay in lane long, and with teleport she can get to allies fast and back into the lane without losing too much xp in the beginning.
She is a support/damage-dealer, to say it like that. Spirit Visage is great for her to be able to get away from ganks since she gets increased healing on herself. Mejais Soulstealer is always great since you will get stacks, always. The usual AP items is a must for increased damage and healing. No need for defensive items here.