Greater Mark of Insight is going to be pretty helpful during early game harassment. Sona is a supportive unit that helps control a lane if used properly. At level one, with Hymn of Valor, she is going to have a pretty good harassment, especially against squishy characters. Also, her Hymn of Valor harassment will help activate her passive: Power Chord. If used properly, in a sequence of: Hymn of Valor harassment (this one obtaining her passive) and then hitting someone with Power Chord, the enemy will take a great amount of damage. Since her passive deals additional magic damage and having it doubled from Hymn of Valor, the Greater Mark of Insight runes will allow more damage to go through. During late game, when Hymn of Valor is going to do lethal damage, the Greater Mark of Insight runes are going to be extremely necessary for the enemy team to damage a huge amount of damage.
Greater Seal of Clarity is not going to be as useful during early game because of it giving mana regeneration per level- the mana regeneration starts off pretty low. However, late game when you reach about level 11, the runes are going to make a huge difference and allow the spamming of skills. When Lich Bane is obtained, the spamming of all of the skills is going to be detrimental to the opposing team, especially when it comes to taking down enemy turrets. Also, having this mana regeneration will allow more laning sustainability.
Great Glyph of Celerity is very similar to Great Seal of Clarity... that it would be extremely helpful during early game harassment and farming. However, late game, again when you are at approximately level 11, the helpfulness with be shown very well. The cooldown reduction is going to be helpful when it comes to having to heal allies with very low health, dealing massive damage with Hymn of Valor, escaping with Song of Celerity, or constantly immobilizing the enemy team with Crescendo during team fights. Also, with skills being spammed this easily, Sona doesn't need a target so it is much easier for her, Lich Bane will be easily activated and more damage output will be made.
Greater Quintessence of Potency is going to be helpful throughout the game, because it allows more damage to be dealt during early game harassing with Hymn of Valor and late game team fight damage output, allow you to heal back more health, and do more addition damage with Crescendo.

Doran's Ring- Doran's Ring has three crucial effects that are a must need on Sona: mana regeneration, health, and ability power. Early on in the game, the runes won't be really helpful, so you are really going to need the mana regeneration from this item to continuously harass with Hymn of Valor and Power Chord. The ability power from Doran's Ring is going to strengthen the harassment and healing that will constantly be going on. Sona is an extremely squishy champion, so she going to need the 100 health bonus from Doran's ring to last longer during the laning stage.
Philosopher's Stone- Philosopher's Stone gives an early on, very needed mana regeneration and health regeneration. Although Sona has a heal with a low cooldown, it does heal as much as you would want, so having some additional health regeneration is going to help, especially when enemies target you for being squishy and being a support. The mana regeneration is going to be extremely helpful during the early game stage when spamming skills for harassment and sustainability is going to be needed. Also, since Sona is meant to farm and sit back and allow carries and other late game, higher damage outputting champions, the extra gold from Philosopher's Stone will help you obtain all of the needed items from this build.
Heart of Gold- Since stacking Philosopher's Stone is no longer allowed because of the recent patch, you can still get Heart of Gold to change that and get even more gold. Also, Heart of Gold gives some more needed health during the early game stage. This will allow you to sit back more and try not to get minion kills and allow your teammates get the kills.
Mejai's Soulstealer- Getting stacks on Mejai's Soulstealer is extremely easy for support characters, especially Sona. Just getting one basic attack, hitting with Hymn of Valor, healing or speeding up allies, or stunning the enemies will get you a stack. Getting these stacks will help with the late game damage output with Hymn of Valor and Power Chord. Also, during team fights, Crescendo will do a large amount of additional damage to its base damage, so you would start the fight off with a large amount of damage output and stunning- similar to Annie's Tibbers stunning nuke. Also, getting full stacks on Mejai's Soulstealer reduces the cooldowns on skills, so even more spamming of skills will happen, and activating Lich Bane will be much easier (when you get it). Also, having a shorter cooldown on Crescendo is going to make team fights easier and allow for more team fights in a shorter amount of time.
Ionian Boots of Lucidity- Getting completed shoes earlier on is not as needed on Sona as it is on other champions, because she has Song of Celerity to get her to places much faster. The best thing amount Ionian Boots of Lucidity on Sona is the cooldown reduction. This is yet another item that will help activate Lich Bane, have more healing, and have a large damage output.
Archangel's Staff- Archangel's Staff is going to be great for the mana that is gives and the great amount of ability power it gives. Since Sona literally spams abilities nonstop, getting the full stacks from Archangel's Staff will be extremely easy to get. This is going to allow even more damage output to be made.
Lich Bane- Lich Bane's ability power along with its ability is going to be really great with Sona. Since she spams abilities, Lich Bane will be activated nonstop, allowing Sona to take down turrets and destroy enemy champions quickly.
Rabadon's Deathcap- Rabadon's Deathcap is always good to have on champions that use ability power. Since it gives 140 ability power and 30% increased ability power, Sona will have a great damage output and healing.
Void Staff- Void Staff gives a good amount of ability power that will be increased by Rabadon's Deathcap. Also, Void Staff gives some magic penetration. When you complete the build, the enemy team is either going to build against you, by getting magic resist, so getting this item with ensure a good amount of damage is still dealt to enemy champions.

The first skill that you are going to want to max first is Aria of Perseverance. Afterall, Sona is a support champion, so getting heal first will allow for you and your laning partner to stay in lane for a longer amount of time. Since you are a squishy support champion, you are going to be targeted nonstop, because the other team does not want you to keep allies alive. So, you are going to need the heal for your allies and for yourself.
The second skill that you are going to want to max, even though you put your first skill to it, is Hymn of Valor. Hymn of Valor allows Sona to harass very easily and to control the lane quickly. However, you should be careful when harassing around minions, because you might take on a lot of damage just from them. A good idea to do with Hymn of Valor is to activate Power Chord in the healing fountain. This way, as soon as minions and enemy champions arrive to the battlefield, you can use Hymn of Valor and then the Power Chord to do a lot of damage and have the damage from Power Chord doubled.
The third skill that you are going to want to max is Sona's ultimate ability: Crescendo. This ability shoots out in a skill shot, so try not to miss when the time counts. This skill has a pretty decent amount of base damage for a support unit's ultimate, plus 80% ability power. This skills causes the enemy champions to dance for a short duration, which is considered a stun. The good thing about this skill is that it can be used offensively and defensively. To help a dying ally being chased down by enemy champions, you can used Crescendo to stop them in their tracks, and then use your other two escaping abilities (Aria of Perseverance and Song of Celerity) to help the enemy get away and not get killed as easily. Offensively getting kills is to use Crescendo on a fleeing champion with low health or stun a team in a team fight.
The final ability to max is Song of Celerity. Level 1 Song of Celerity is all that is needed for a very long time, because your other skills are needed much more for harassment, stunning, and sustainability. This ability is good for three things: pursuing, escaping, and getting stacks on Mejai's Soulstealer.

The two skills that you are going to really want are Clairvoyance and Flash.
Clairvoyance is a skill that most support champions are supposed to get because any other summoner skills aren't as useful on support champions. This skill is really good for dodging possible ganks, spotting and ganking enemy junglers, seeing where enemy champions are going to be laning and what items they chose, and to view low health champions for allied champions to kill that have long range or full range ultimates, like: Ashe, Ezreal, Twisted Fate (not as much because his ultimate gives vision of enemy champions), Pantheon (not as much because his ultimate is easy to dodge), Lux, Caitlyn, or Gangplank. (Sorry if I missed some)
Flash is a very needed skill on a Champion like Sona. She has two escaping abilities, but they are very weak escaping abilities: Crescendo and Song of Celerity. Flash is sometimes a guaranteed escape or chase. Sona can even flash in to heal, use her ultimate to help kill, or use Hymn of Valor for a kill. However, I truly recommend that Flash is a summoner skill to get.
Another summoner skill that can be used instead is Clarity. Clarity will be needed early on, if you decide to take it. This will allow you to spam Hymn of Valor and Aria of Perseverance more often to have more harassing and lane sustainability. If you decide to take Clarity over Clairvoyance or Flash, take one mastery point out of Mystical Vision or Blink of an Eye (depending on whether you don't use Flash or Clairvoyance) for Insight.

The only things that can be said about team work with Sona is when to use Crescendo during a team fight or when to use to help a dying enemy.

The great thing about Sona is that she does not need to worry about farming a lot. Her gold source is from getting assists everywhere. Sona should be used to weaken enemy minions for allied laners or to defend the farmer that you are laning with so they can farm well.

Pros

Extremely easy to play with only one skill shot and no point and click shots
Great sustainability in lane and during team fights
Great early game harassment
Great at controlling a lane
Large damage output late game
Has an ultimate that can stun a team
Can spam abilities easily

Cons

Very squishy
Targeted a lot
Very poor escaping
Has to be aware of mana a lot early game
Will be blamed for nearby deaths