Not Updated For Current Season
This is still getting the kinks worked out so dont take it too seriously.
I just started playing Sona maybe 2 days ago. Had about 10 matches and only lost 2. (I lost my first game with her cuz i wasnt sure how to really play her, you know how that goes, and i just lost my second game after a 52 min 4v5) My average score for games is prolly 3/2/15. Not spectacular but shes great for support and harassing early on.
About the items:
Once your level 18 you should have up to at least Innervating Locket. From my experience you can practically cast endlessly without noticing an issue with mana, so feel free to heal away! Im sure your allies wont mind at all.
You dont necessarily have to get Mejai's Soulstealer. A lot of times i wont get to 20 stacks before the games over or if i do it ends shortly after. If your getting beat down you could prolly pass it up and get something else on the list.
Depending on who your playing against you could switch up Rylai's Scepter with something else. For instance if your taking a lot of magic damage try Abyssal Scepter, or if your taking physical damage maybe frozen heart. Most of the time i just use the items in the list to build up health and try to stay away from the fighting and heal from afar and buff teammates appropriately.
I start out with Hymn of Valor to harass whoever im laning. Not long you can get a heal so you can start pushing more. This is great when your laning with a tank or fighter, since they wont need a heal right off the bat even if they get hit. (Pantheon seemed really great for some reason in my last few games, i usually got him to 3-0 within 10-15 mins. Getting your teammates fed is just as good as you getting kills. Remember this is a team game =] )
One last piece of advice... Dont quit before the games over! I had a game where we had 3 surrender votes, thankfully they all failed. After they pushed into our base and killed one of our inhibitors we grouped up and aced their team. Pushed all the way into their base when the respawned we were able to fend them off again since i was there providing heals and mana when needed. Theyd rush out attack us 1 at a time and fail, run back to heal, by the time they came back i had my entire team back up to almost full health.
You will be targeted first! Stay in the back best you can, but not out of range of your spells. Dont get discouraged if you get ganked early on. They are looking for easy pickings and Sona fits the bill perfectly, especially early on. I got ganked 3 times in a row early one game but was able to keep my allies alive when needed and we pulled off the win and i finished 5/5/21. So dont surrender unless they are tearing through your towers like they are paper and ur allies arent even touching them.
Thats all i got for now i think. Ill update this as i see fit as i play her more. Thanks for reading all this if ya did!