Flash is generally a core spell, there's no reason to switch it out for anything. Exhaust tends to be a stronger pick throughout the game, as it gives you a massive damage debuff that is particularly effective against high damage assassins like Zed, but picking up Heal while your carry picks up Barrier can also be a strong option. Ignite can be picked up simply to give you early pressure or against a high sustain team or against a high sustain support like Soraka, if necessary.
Sona uses Summon Aery extremely well, it's the best option you can run on her. The other masteries are largely personal preference, though I will say that Scorch really adds to your early game damage, so it's much more worth it than the other options available to her. I often switch between using Resolve and Inspiration as my second trees, depending on the game. Both have runes that are beneficial to you, so feel free to pick whichever you prefer.
Alternate Options: Sorcery & Inspiration Sorcery & Precision
I'm a support player from the North American Server. I play primarily Nami, Karma, and Lulu, but I sustained around a 70% winrate with Sona last season. Sona is one of my favorite supports in the game. She came out shortly after I started playing League of Legends and I recall being really excited to play her in her first free week.
Sona definitely has an interesting history in this game. She spent a fair amount of time being one of the strongest supports in the game, despite having her base stats nerfed such that she was basically the same as a minion at level one. She has since fallen largely out of favor, though the more recent change to her ultimate did bring her back for a short period of time. Sona is still a pretty decent pick overall and is a good pick for players new to the support role as she offers a lot of utility and requires a lot less skill to play than many other champions.
Pros +decent sustain +very strong poke +good engage and teamfight potential +low item dependency +fairly good at kiting +
Sona is a generally solid support at the moment. She has good damage potential in lane, decent sustain, strong engage, and kiting power. She is also fairly easy to play in a mechanical sense, which can make her a good pick for newer support players. She is also a force to be reckoned with in teamfights if you know how to engage with Crescendo properly.
Sona is extremely squishy in lane, this makes her somewhat vulnerable to poke or high burst supports. Aria of Perseverance has also never been particularly mana efficient, though it's better than it used to be. This means she has to be careful just how often she spams it, or she will be very vulnerable. Her primary damage ability, Hymn of Valor, is not targeted (it simply auto-targets whoever is closest), which can be a blessing or a curse depending on how far back you are forced to be.
Cons -extremely squishy early -can have resource problems in lane, especially if Aria of Perseverance must be used often -ability to utilize Crescendo post six can make or break a lane with Sona.
Power Chord: This makes Sona an excellent damage dealer in lane and also has a lot of utility due to the different functions based on the last ability used.
Hymn of Valor: This skill is your primary damage ability. It is not targeted and prioritizes champions, so you just need to ensure you are in range of a target to hit them with it. It also gives you and nearby allies on hit magic damage on the next basic attack within three seconds. It makes Power Chord deal an additional 40% damage when it is the last ability cast, which is quite good for damage early on.
Aria of Perseverance: This skill gives you a decent amount of sustain and is a convenient way to apply shield triggered buffs to your entire team later in the game due to the small shield it applies to all allies within range (for one and half seconds). It applies a damage debuff to Power Chord when it is the last ability cast (that stacks with Exhaust), which can be fairly useful for winning trades if necessary.
Song of Celerity: This makes kiting extremely easy. It also gives you the ability to slow enemies with Power Chord, which can be a nice follow up for ganks before you have Crescendo.
Crescendo: This skill is very effective for initiating fights. The skillshot is wide and travels rather quickly, so it can be combined with Flash for a pretty strong engage. It also includes a passive that decreases the cooldowns of all of your abilities, in addition to any CDR you have from other sources.
Currently, maxing Hymn of Valor first is the best option on Sona as it gives her strong poke early game. I usually prefer to max Aria of Perseverance second, though it can be maxed first if the match up is difficult. In this circumstance, I still recommend taking three points in Hymn of Valor before you max Aria of Perseverance, so as to not lose all ability to harass. I then max Song of Celerity last, as you really only need one point to get the slow on Power Chord. You can even forgo points in Song of Celerity until level 8 or level 12, if you wish, though it's not my own personal preference. Crescendo should be taken whenever it's possible to take it.
Starting Items Either of these are fine starting items for Sona. I generally prefer Spellthief's Edge as it fits Sona's playstyle better, but Ancient Coin is okay if you are in a difficult match up and want some extra self-sustain.
Core Items As some type of warding item is core on supports in general, mentioning it seems almost redundant. However, the health and extra AP Eye of the Watchers provides is quite beneficial on a squishier support like Sona, while Eye of Frost can be a nice option to sit on if you want to get another item earlier. Redemption is a core buy on most supports and Sona really benefits from everything about the item. Oracle's alteration is another core support item, that should be bought fairly soon after hitting level 9, as it allows you to gain vision control easier, which is important for keeping an eye on objectives such as Baron Nashor and Dragon.
Defensive Items These are the best defensive purchases for Sona currently. Mikael's Blessing is very strong against compositions with a lot of hard CC, it can save an ally from an untimely death if the active is timed properly. Locket of the Iron Solari should be purchased against teams with a lot of Magic damage, especially if it's AoE, but is strong against any high burst team compositions due to the shield active. Randuin's Omen still remains the most efficient armor item for a support, as it provides health for its armor. This means the item needs very little health within the build to be effective, while Frozen Heart requires more health within the build or the armor is largely wasted. Zz'Rot Portal is a pretty decent purchase if your team is getting pushed in or if you want to start an AFK push in one of the lanes to force the enemy team to send someone to deal with it. Banner of Command is sort of a budget Zz'Rot Portal and, while cheaper, is more situational as the promoted minion is only immune to magic damage.
Offensive Items Sona largely doesn't need to purchase offensive items. However, there are a few items that can be beneficial to purchase on her. Ardent Censer gives her a good chunk of AP, CDR, and movement speed. However, its primary appeal is the passive it applies to your healing, which gives targets healed or shielded attack speed and on hit magic damage. This makes it useful to rush with ADCs who utilize attack speed well, especially if they would normally get Runaan's Hurricane as one of their core items. It also further increases her healing values, which is great later on in the game. Zeke's Convergence is a strong purchase if you wish to further buff an ally's damage, her ultimate is on a fairly low CD later in the game and she can fairly easily ignite the frost storm.
Situational Purchases These items are extremely situational, but can be useful in or against certain team compositions. Banshee's Veil is more of a selfish purchase, which I would get against champions such as Syndra or LeBlanc to mitigate as much damage as possible. Frozen Heart can be a good purchase against attack speed dependent champions or against those who utilize attack speed as part of their damage ( Yasuo, Lucian). However, the lack of health in the item itself means you need to itemize health to make the best use of its armor, which can be tricky on a mage support like Sona. Spirit Visage is an okay purchase if you are dealing with a lot of magic damage and simply need a lot of MR to deal with the problem. Knight's Vow is an interesting item, but everything it does is reduced by half on a ranged champion like Sona, so it's definitely situational and I feel as though other items tend to be better for her. Twin Shadows is a fairly good option against stealth champions, as the ghosts will chase champions who enter stealth after it's activated. It's also a good scouting tool, if needed. Spellthief's Edge better than Ancient Coin in lane. This does not make it a poor purchase for her overall, but it tends to be best in harder match-ups rather than as a general purchase.
Largely Sona's function in lane is to provide poke, sustain, and engage. She also has decent disengage, which makes her pretty flexible. Mostly you want to poke enemies with Power Chord. Exhaust should mostly be used in conjunction with Diminuendo or Tempo to disengage or to lower an enemy's damage in a trade, though it can be used to chase targets (Tempo will usually allow you to forgo this). Crescendo can be a powerful engage tool in lane post-six when combined with Flash. It can also be used to force enemy champions to back off or to foil a turret dive, as the width of the skill shot makes knocking up grouped targets easy.
Largely your function steers away from dealing damage and more towards buffing and sustaining allies and debuffing enemies, however you will often become a key initiator with Crescendo. You'll want to use Diminuendo on high priority targets like their marksman or assassins and Tempo to chase targets down if necessary. You'll probably end up using your non-ultimate abilities in rapid succession to buff your allies and debuff enemies as needed. Exhaust should largely be used for peel, chasing shouldn't be a problem if you're using Song of Celerity as needed.
Remember that Power Chord does different things based on what ability was used last before attacking an enemy. Hymn of Valor will make you deal 40% extra damage with Power Chord, plus its on hit magic damage if you proc Power Chord within its duration. This will be your primary source of harass early game. Aria of Perseverance causes Power Chord to reduce the enemy's damage by 25% (+4% per 100 AP) for three seconds. This stacks with Exhaust (multiplicatively) and can be used in a trade to reduce an enemy's damage by a fair amount if necessary. Song of Celerity causes Power Chord to slow enemies by 40% (+4% per 100 AP) for two seconds. This is a pretty good option to use when you want to follow up or prepare for a gank, as the slow is fairly strong and the movement speed from Song of Celerity can help you and your carry to get in a better position.
Should I build items like Lich Bane?
In my personal opinion, no, Sona should not be built as some sort of budget AP carry with a few support items. She does not need items like Lich Bane to remain relevant in lane (one of her perks and why you generally don't see pros buying it on her) and it mostly just wastes gold that would be better spent on items like Redemption and Locket of the Iron Solari. Her job later on in the game is not really to deal a lot of damage, which just makes building a lot of AP a bit of a waste. Sona also needs to get fairly up close and personal to engage, so being squishy with a bunch of AP items is not particularly desirable. If you want to play a support like that, Zyra and Brand are far better choices than Sona.
Sona works well with most carries and team compositions, however champions such as Varus and Lucian can utilize the extra on-hit damage from Hymn of Valor quite well and are able to follow up her poke with poke of their own. They also have strong follow up ultimates to chain with Crescendo and use movement speed handily. In general, Sona works well with champions who can follow up on her engage easily.
Sona is extremely squishy early, which can make dealing with strong poke champions like Brand fairly difficult. She especially struggles against champions who can poke her from outside her own range or force her under tower often. Caitlyn can easily avoid most of Sona's poke due to her superior range, which can make her an annoying champion for Sona. Lulu can nearly equal Sona in poke, has a strong enough shield to block Sona's poke if she uses it properly, and has strong point and click CC abilities that can make things difficult for you.