Pros
[*] Good poke
[*] Good Sustain
[*] Auras
[*] Great Ult
[*] Very Flexible (can build to fill almost any role) Damage, Peel, Initiation etc.
[*] Easy to Use
[*] Great level 6 all in

Cons
[*] Squishy
[*] Mana hungry
[*] Hard to gank for pre 6
[*] Weak teamfight compared to some other supports when ult is down

Synergy
Sona works well with almost every AD. She works very well with other champions that spike at level 6 ex. Vayne or Twitch.

Counters
Sona doesn't really have any hard counters. Her hardest lanes are lanes like Leona/Thresh/Blitz that can jump on you and burst you down because you are so squishy be very careful with all three of these champions at level 2 because that is when they are really strong.

Also you can argue that Annie is also a hard lane because she can do a lot of the same that you do though you have sustain where she has more burst.

Flash is your number one summoner on Sona because she does not have an escape. Also it allows you to be able to flash Crescendo for initiation in a team fight. For that reason flash is always taken over ghost.

Ignite or Exhaust. These are the contenders for your other summoner slot. Usually I take ignite to help get kills in lane so it is easier for my team to carry. However, exhaust is also an option and in some situations is better than ignite. For example if you are going against a Vayne I almost always take exhaust because she had to get in range to do damage and it slows her attack speed which is what many Vayne players stack in the recent/current meta.

Runes can be quite varied on Sona.

I always take Flat armor yellows. I do this on almost every champion and I cannot think of an example off the top of my head where these would be different.

For blues i usually take Mr per level because ap mid laners like to roam around lvl 7-9 and mr per level are about as good as MR flat but they give more stats later in the game. However I make exceptions to this rule quite often. If I facing a heavy magic based support such as Annie/Fiddle/Zyra I take flat MR to help mitigate their harass so I can win trades easier. I don't run other blues on Sona because the Mr is so important because she is so squishy.

I like to take Flat Armor marks on Sona and almost all the supports I play. I find that with the marks and the seals I start with a bonus 21 armor which makes early trades go heavily in your favor because most ADC run ad runes. I used to take MR pen but I really like taking flat armor now although they are still a good choice.

Quints are where it comes down to you as a player. I normally take MS or Gold quints but health is also a good option as well as ap or Mr pen. It really comes down to what you want to take but I like to take MS because it allows me to base and get back to lane faster, ward and get back to lane so I miss less xp and it allows me to contest objectives with my jungler really easily post 6.

I like to run 0/9/21. The 21 in utility helps with my gold income especially with bandit combined with Spellthief's. Intelligence is also really nice because it gives me 40% CDR faster and it is not completely wasted if I buy 40% CDR because it reduces active item cooldowns. The MS from the utility tree is also really nice. The nine points in defense with block and unyielding help complement you high early armor to take less damage and make all ins easier as well as harassing. Health and health regen is always nice no matter what champion you play and helps mitigate trades further by helping regain lost health and the flat health helps with enemy burst.

These are not the only masteries you can run on Sona. I have seen people go more into the defense tree if they need to do more initiation and tanking or more into the offense tree if they want to do more damage. It is really up to you and how you want to play Sona as she is a really flexible champion.

Power Chord
After 3 ability casts, Sona's next basic attack will deal 6 + (7 / 8 / 9 / 10 / 15 at each level) (+ 20% AP) bonus magic damage, with an additional effect depending on the last basic ability cast.

Hymn of Valor RANGE: 700 COOLDOWN: 7 COST: 45 / 50 / 55 / 60 / 65 MANA
ACTIVATION: Sona sends out bolts of sound that deal magic damage to the two nearest enemy units within range, prioritizing champions.
MAGIC DAMAGE: 50 / 100 / 150 / 200 / 250 (+ 50% AP)
STANCE - PERSISTENT AURA: Sona plays the Hymn of Valor, granting nearby allied champions within 1000 range bonus attack damage and ability power.
POWER CHORD - STACCATO: If this spell was last cast when Power Chord is ready, Power Chord deals double damage.
ATTACK DAMAGE & ABILITY POWER: 4 / 8 / 12 / 16 / 20

Aria of Perseverance RANGE: 1000 COOLDOWN: 7 COST: 60 / 65 / 70 / 75 / 80 MANA
ACTIVATION: Sona heals herself and a nearby allied champion with the lowest health percentage, granting both of them bonus armor and magic resistance for 3 seconds.
HEAL: 40 / 55 / 70 / 85 / 100 (+ 25% AP)
ARMOR & MAGIC RESIST: 6 / 7 / 8 / 9 / 10
STANCE - PERSISTENT AURA: Sona plays the Aria of Perseverance, granting nearby allied champions within 1000 range bonus armor and magic resistance.
POWER CHORD - DIMINUENDO: If this spell was last cast when Power Chord is ready, Power Chord will also reduce the target's damage output by 20% (+ 2% per 100 AP) for 3 seconds.
AURA ARMOR & MAGIC RESIST: 6 / 7 / 8 / 9 / 10

Song of Celerity RANGE: 1000 COOLDOWN: 7 COST: 65 MANA
ACTIVATION: Sona grants nearby allies increased movement speed for 1.5 seconds.
MOVEMENT SPEED: 4 / 6 / 8 / 10 / 12% (+ 2% per 100 AP)
STANCE - PERSISTENT AURA: Sona plays the Song of Celerity, granting nearby allied champions within 1000 range a flat movement speed bonus.
POWER CHORD - TEMPO: If this spell was last cast when Power Chord is ready, Power Chord will also slow the target by 40% (+ 4% per 100 AP) for 2 seconds.
FLAT MOVEMENT SPEED: 4 / 8 / 12 / 16 / 20

Crescendo RANGE: 1000 COOLDOWN: 140 / 120 / 100 COST: 100 / 150 / 200 MANA
ACTIVE: Sona plays an irresistible chord in a line, dealing magic damage to enemy champions and stunning them for 1.5 seconds, forcing them to dance.
MAGIC DAMAGE: 150 / 250 / 350 (+ 50% AP)

The abilities on Sona are quite straight forward. You max q in lane for the harass. Max w second for the sustain in lane(in certain matchups you may want to max w first or at least put a couple of levels in it if you ad is having a hard time farming). For example if you are laning against Cait and your ad is a Vayne and Vayne is constantly getting poked/not the best positioning/not sure how to play the role. However this is a trade off if Vayne is only ~10-15 cs behind and can deal with the lane maxing q is better so you can all in at lvl 6 when Vayne hits her power spike.
(Maxing q or w depends on matchup and how you are doing in the matchup it is something that you have to get a feel for through many games bot lane).

Now I want to talk about e. I don't normally take a point in e until lvl 13 because it only gives a 4 ms with one point and ~16 ms for 1.5 seconds if the champion has 400ms already. However this can also be situational mostly because of the passive which I will talk about next.

MOST IMPORTANT THING IN THIS WHOLE GUIDE: Sona's Passive Power Cord
Sona is an easy champion to play for the most part. However her passive is what makes you able to carry a game. You probably think that is a bold statement and that her ult is what lets her carry games(while it is and can win a fight the passive is extremely important in fights) and you are partially right a good ult can win you the game plain and simple but people underestimate her passives.

Here are Sona's passives again:
POWER CHORD - STACCATO: If this spell was last cast when Power Chord is ready, Power Chord deals double damage.

POWER CHORD - DIMINUENDO: If this spell was last cast when Power Chord is ready, Power Chord will also reduce the target's damage output by 20% (+ 2% per 100 AP) for 3 seconds.

POWER CHORD - TEMPO: If this spell was last cast when Power Chord is ready, Power Chord will also slow the target by 40% (+ 4% per 100 AP) for 2 seconds.

Now let me explain when you might use these passives and see how important they are.

Staccato(Double Damage): You normally use this when you are harassing the enemy in lane or if you are poking the enemy under turret in a siege. Early levels this can help you get to a point where you can completely zone their laners from the wave if you trade really well or they make too many mistakes.

Diminuendo(Reduce enemy damage output): This is probably the most important of the passives. You use this in skirmishes and team fights. It is important to try and land this on an enemy carry without putting yourself in a bad position. Now if you are the initiator try and get two stacks of your passive ready for when you flash Crescendo so you can hit w then put this on an important carry such and their ADC or almost better if you can put in on a burst caster before they get their bust off it could save one of your carries lives so they can keep doing damage. This also allows you to heal them in extended fights so they survive even longer.

Tempo(Slow): This is also a very important passive. In the mid game putting a point in e so you can get this passive is very good versus mobile opponents like Riven/Lee/Nidalee so it is easier for your team to catch and kill them. If you are not initiating late game this is probably the passive you are going to want to use if only because it can help your carries kite. For example if Jarvan IV uses his e q combo to jump on your carry it can be very effective to slow them so your carry can kite them but it could also be a good idea if they are just trying to assassinate your carry to use Diminuendo(Reduced Damage) to stop their bust so you carry can survive or if other members of you team can also peel for your carries.

In summary you should really only be using your q passive if you are a damage source because you got feed. You should use your w or e passive depending on the role you are fulfilling in your team.

I like to start with warding totem on Sona. It helps protect my lane from ganks in the early levels before I back to get wards or a Sightstone. You can also go lens if you prefer to have bush control or if you want to clear a path for your jungler to gank. However I recommend getting Lens as soon as you get a Sightstone or around level 7/8 as Lens becomes far stronger than totem at lvl 9 IMO and if you have a Sightstone you have no need for a warding totem any longer.

This different for other supports though. For example if I am playing Thresh/Blitz/Leona where I want to have bush control early to make play I almost always take Lens for bush control to help zone them or make plays on them.

I have never taken to blue trinket because it has a cooldown that is far to long for what it does. Besides you should not be face checking bushes you should know where the enemy is with your vision and if you don't you have a problem because either you or your teammates are not warding properly(at least until late game when pinks and lens are more prevalent especially around objectives however at this point you should have a Sightstone to ward if you are worried that they are in a bush).

I start Spellthief's because I find it to be good with Sona's and my playstyle. It also tends to give more gold than Coin if you can get harass on your opponents. It is also better than Relic because Relic now only executes on melee so it is harder to use the passive.

I like to upgrade Spellthief's to Frostfang as soon as possible to increase my gold gain.

After that depending on how my lane is doing I get either a chalice or a Sightstone. I usually get chalice to spam my harass and heal my AD. I would get Sightstone if the enemy jungler is camping bot to help protect against ganks and to have brush control.

This is usually where my build can change a lot. It depends a lot on the team comp that I am playing with. I like to get Boots of Lucicidty into Mikaels/Talisman. I do this for the CDR and mana to help me spam heal and get my ult up more often to make plays as well as the actives on these items.

Another option is if you are wining very hard to go for raw damage like a Deathcap/Zhonya's/Deathfire Grasp. This is usually only if your whole team is stomping them.

If your team needs more tankyness it can also be a good idea to go Randuin's/Spirit Visage/Locket/Abyssal. This can allow you to stay alive longer in fights and help peel for your carries. I don't recommend picking Sona if you need a tank as other supports are better for this such as Leona.

Sona is such a flexible champion with her kit that you can fill in just about any role that you team needs so long as you build the right items.

IMPORTANT: Regarding sightstone as of right now it allows you to place three wards without upgrading it to Ruby Sightstone unlike in S3. So don't upgrade it unless you really need the HP because it only gives 475 gold worth of health for 600 gold.

Your preference of boots. Usually Mobility or Lucidity.

I like Athenes or Mikaels for the mana regen,I almost always get an early chalice.

Talisman/Locket/Sightstone are all good items also.
Additionally depending on the situation you could get
Twin Shadows/Banner of Command

All the items you build on Sona depend on your team comp and the enemy team comp as well as how well you are doing or if your behind. However Sightstone is always needed and I recommend getting Athenes or Mikaels for at least one of your items.

At base level your starting skill immediately upon entering the game and cast it for a stack of your passive then you can buy your items and cast it again so when you go to lane you have full mana and two stacks.

Level 1 you shine in a lot of matchups (because q can hit both champions) however you have to be careful of hard engage. If you enemy is Leona/Blitz/Thresh with Draven/Corki be careful of harassing make sure you have bush control or that your enemy can't engage on you when you poke or they can probably burst and kill you or at least make you burn flash. Although don't be afraid to poke if they are attacking you ADC because they probably can't attack you both at the same time (except Sona because of q or aoe damage).

Post 6 is when Sona really shines. If you are with someone that all ins like Draven/Vayne this is the perfect time, if you are about even or ahead, to all in for a couple of kill especially if you took ignite. If you can't engage for some reason (losing/lot of disengage Janna/Zyra) you should try to roam mid and flash Crescendo for a kill mid to help your team snowball. If you can't do that follow you jungler and try to steal some buffs and get some deep wards. Going top is also an option if you have to but that is not really an option unless you base first and if your AD is behind it can really hurt them without having any support. This is when you have to carry if you haven't gotten your AD fed yet.

Sona really shines in 2v2 or 3v3. Her q can hit two opponents and her auras are good for fights. Her aoe stun ult is really good for ganks and (if you have a point in it) the passive from e allow you to chase and kill enemies easily. If you can, group early and take objectives post 6 as soon as possible because you are at your strongest point. You are good for dragon fights at this time as well because of the high base damage of q and heals help with enemy poke.

This is where you have to play the best out of the whole game. If you have gotten to this point and you do not have a decisive lead you have to play smart. However DO NOT BE AFRAID TO USE YOUR ULT. I don't know how many people I have seen hold onto their ult only to die or not use it to catch out the person out of position. Understand unless you are in the LCS you are probably never going to see a perfect team fight. If someone is out of position and YOUR TEAM CAN BACK YOU UP!! then use the ult on that one person to burst them, then the fight is a 4v5 and usually your enemies will back off or you can win the fight easy. If you try to do this and you miss your team should probably back off as unless you are ahead you just wasted a lot off CC but your ult should be down to an 80-60 second cooldown by this point.

Sona is a really strong champion at all levels of the game due to her utility and her early game harass. When she is used properly she can win your team the game or lose them the game.

Teamwork is also your greatest strength with Sona because she tends to be weak in a 1v1 but great in skirmishes and team fights.

This is my first guide if you have any points you can think of that i have forgotten let me know in the comments. This guide was mostly created for myself to help think through the thought process I use when I play Sona. This guide was also created in the middle of preseason 4 so things may change with buffs/nerfs/item changes.