Okay I have about 30 minutes to explain as much as I can.
But before I start here are the links to the threads I posted that will help you out to shorten what I have to say here.
Sona's Auto-Attack Reset: http://www.reddit.com/r/RealSonaMains/comments/3w3rs3/useful_mechanic_to_increase_your_poke_and_trades/
Windspeaker's Blessing vs Thunderlord's Decree: http://www.reddit.com/r/RealSonaMains/comments/3w9kfl/windspeakers_blessing_vs_thunderlords_decree/
Okay so now I will try to explain everything as concisely as I can to save time so feel free to ask for explanations, I will get back to it when I can.
Sona is a very versitile support she doesn't have many core items she only really has situational items. The only items I consider core on Sona are: Frost Queen's Claim (or the sightstone upgrade of this if you want to preserve the extra slot) and Sheen
Sona serves multiple functions as a support depending on what you build: Zoning, Lane Bully, Utility, Peel, Engage, Teamfight, Picks, and Burst
I will explain builds later as it will take the most time and it isn't as crucial to understand as a ton of items are viable on her.
Matchup wise there are a ton of supports and a ton of adc's so i wont go too much into the combo's but more about individual champions with a few combos listed.
Soraka: You can burst the adc all you want but no matter how much you poke the adc you can guarantee Soraka will heal it off in no time. Focus Soraka to get her lower before you start harassing the adc.
Leona: You beat her level 1 and 2 but since she is a tank you wont be able to continue harassing her as the game progresses. Unless her adc is Draven, you will want to harass her until she hits level 3-4 then you need to bait out her Zenith Blade and punish the ADC for her miss. At level 6 you have to be extremely careful as Leona's level 6 powerspike is larger than yours is.
Draven: Draven's spinning axes allow him to out trade any damage you will do early on. Harass him as he throws his autoattack at a minion, he wont be able to return damage. Make sure you do it quick enough that he doesn't have time to return with autoattacks because he will punish you.
Leona + Draven: This lane is a horrid lane for Sona. Draven will out trade your damage and Leona will not take damage from you. Level 2 they will likely engage on you in hopes for an early kill and Leona will be looking for every chance she gets to jump in on you or the adc after that. If they hit level 6 before you the lane is over, unless you manage to shut them down early on, ideally through the help of the jungler.
Janna: Janna is a tricky one as she cant out trade you herself but she can make the ADC win trades for her with her shield. See if you can bait out Janna's shield and then harass either Janna or her ADC. You should win this matchup unless the enemy ADC is Caitlyn.
Caitlyn: Dont try to trade with her unless you are sure you can win it. Her massive range makes it almost impossible to get away from a trade clean. Avoid her traps at all costs and punish her for any bad positioning she does. Other than that it is better to focus the support, unless it is a Leona.
Braum: His shield will stop both your powerchord and your ultimate. Bait that out before you do anything. Beware of his passive you will not want to get stunned.
Lulu: Dodge her glitterlance and you win, save your ultinate in fights for when Lulu polymorphs your ADC or speeds up her ADC/herself to prevent free damage. If you are going to ult to engage wait for her to burn the polymorph/speed up. Her ult will save the ADC and is a free extra ~400 health. Be prepared for that when you fight.
Blitz/Thresh: They serve the same purpose to hook you. If you dont get hooked you will be fine. Punish thresh for collecting souls and take the offensive when their hooks/grabs are down. Dont let them get within melee range of you, kite them if they try. Use your E powerchord when needed.
Brand: He is really strong, dodge his W and Q's, try to punish him early. Not much else you can do.
In any matchup if you have the offensive make sure to zone off the enemy from the farm. Bonus if you can keep them out of experience range.
Thats all I have time for now, if I haven't mentioned a champion its because they go even, you win, or they aren't super hard counters to Sona. I will add more when I get my last break in 2 hours from now and again when I get home. Hopefully this helps some.