Not Updated For Current Season
Take one of the side lanes and partner with someone. Tell your partner to not expect heals, as the 40 or so hp you will be able to heal from lvl 2 to 7 is almost nothing.
The main point of early game is to farm the gold for the tear as soon as possible, so last hit with both your normal attacks and the Hymn of Valor spell as much as possible. Of course, if you have the opportunity, always hit enemy champions - if you manage to push them back and not let them farm it's just better for both you and the whole team. Be smart - try not to get to places where you can be easily ambushed or be injured. If you are damaged, heal yourself using the Health potion, don't waste mana on heals. As soon as you have 605 gold, heal your ally, warp back home to get the tear and teleport right back to the battle so you don't lose out on experience.
After you are back, continue what you have already started. Last hitting will become much easier with every level of the Hymn of Valor, so to make it harder again, try to last hit two minions at once. Try to farm as much gold as possible before coming back to recharge your mana. If you managed to save the Mana Potion before coming back the first time, use it now. Hopefully you will have enough gold to get the Sorcerer's Shoes.
Afterwards continue on working on 3 Archangel Staffs. This will give you a lot of ability power which leads to very high damage from your Hymn of Valor and your heals will become insanely strong. Also, since you'll have tons of mana you will very rarely have to go back home. Lich Bane and Zhonya's Ring are optional late game items - if you and your team are good, very rarely will the games last that long. Still, since you will be casting all 3 of your spells very often and your ability power will be very high, Lich Bane will allow you to inflict some damage with your normal attacks as well. And Zhonya's Ring will further increase the amount of ability power you have.
For early game you have the hp from runes and masteries which will increase the number of the health bars above your character and make your enemies less interested in you as more hp means harder to kill. Try to avoid receiving damage as much as possible. If you do receive some and are lacking 3 or 4 bars, do not hesitate to use the Health Potion, as the less hp you have, the more delicious you will seem to enemy champions. If you get attacked, try to run away with Song of Celerity, heal yourself and use Flash if necessary. If the enemies get stupid enough to follow you under your tower, cast Crescendo on them if you have it to make them pay.
For team fights, there is no need for Sona to be in the front lines. Just keep a distance and cast all 3 of your spells as soon as they become available. If the enemies suddenly approach you, cast Crescendo and move back to a safer distance, let your team deal with them, but do not go away - they need your heals and your damage helps a lot. Watch out for fleeing severely injured team members - your Song of Celerity might help them escape. Depending on the situation you might want to follow them and heal them, or maybe even use Crescendo on the fleeing enemies.
The main point is trying to not get involved where all the damage is dealt. Sona is a weak hero when it comes to survival. However, if you stay safe helping your team from a distance and the team is smart enough to teach anyone who is willing to try and hurt you a lesson, you'll be ok. Therefore in this build I am not going for survival items but ability power items instead. And since your ability power will be so high, there is no need of getting it's runes as a few more ability points are pretty much worthless. And since your health is very low, that tiny bit more helps quite a lot.