• Power Chord makes Sona a good early game harasser.
  Charging Power Chord fully at base at the start of the game can lead to an advantage.
• Sona's Hymn of Valor is amazing, it does tons of damage, and is great for harassing the enemy. Don't worry about it hitting creeps, it just helps push the lane a little, you shouldn't use it though if your ONLY going to hit the creeps and not the enemy. Combine this with your power cord and hit the enemy champ with it.
• In a battle using Aria of Perseverance along with Power Chord to hit the enemy's AD carry lowers their damage output and can help turn the tides and win fights.
  Use Aria of Perseverance when an ally is low on hp or is being focused , it gives them a nice heal and also gives them bonus armor and magic resistance.
• Do not underestimate the movement speed buff of Song of Celerity. The passive aura combined with its AoE active can give a very large speed boost to Sona's team. This combined with its slow from Power Chord can make it very difficult to catch/run away from her and her team.
• Sona makes an excellent healer, Crescendo is best used when the whole team can take advantage of it, or when your AD is in need of saving, don't hesitate to use it to save them, they are the most important role in the game and you need to keep them alive.
• Without a teammate around you need to be extra careful,Sona isn't defenseless but be careful at all time and wary of ganks or dives from the enemy team.
  
  Tip: During laning phase, make sure to keep either her Hymn of Valor aura active if your lane partner wants to be aggressive and last hit a little bit better, or her Aria of Perseverance if you are more on the defensive.

So for items you can go more support by getting more things like Zeke's Herald or Randuin's Omen, but for this build were going to go a bit more on the AP side of things.

You first want to start out by getting an Amplifying Tome and 1 Sight Ward. The ward should go into tri brush if your bottom or if your top for some weird twist of fate you should put it in the small bush in river at the tip closest to baron to get the most use out of it. ( The amp tome gives you a lot of AP early so make sure you harass the enemy with your Q Hymn of Valor )

Your going to build your tome into a Kage's Lucky Pick and then get a Philosopher's Stone while still getting Sight Wards. Your next buy you will be getting your Ionian Boots of Lucidity and more wards. ( Getting a Kage's and Philo will help you with gold problems because you wont be getting cs, also with Kage's and Philo you'll have fairly good mana regeneration and poke. Getting Ionian Boots of Lucidity really helps because you need that cool down for your ult in team fights and for all your spells. )

Next you'll be building a Hextech Revolver and also buy a Ruby Crystal and more wards. ( always get wards when you go back! )

You should have a good amount of gold from your Kage's Lucky Pick and Philosopher's Stone by now and you'll be finishing your hextech into a Will of the Ancients, your next bug buy will be turning your Ruby Crystal into an Aegis of the Legion. ( Getting a Will of the Ancients helps not only you but your whole team and especially your AP mid, getting an Aegis of the Legion helps everyone, this is a HUGE item, you should always be getting this item on Sona unless your top or jungler says they'll be getting it. )

And finally for your last items you'll be turning your Philosopher's Stone in a Shurelya's Battlesong and your Kage's Lucky Pick into a Deathfire Grasp. And never forget to ALWAYS buy wards, no matter what, because wards win wars. ( By getting Shurelya's Battlesong you can run away from the enemy team or run in to get off your ult for an initiate. And lastly by getting a Deathfire Grasp you'd want to use this on the champ who has the MOST hp on the enemy team, DFG helps burst them down and helps your team kill them quicker. )

Note: also getting an Oracle's Elixir to clear wards is great and can help your team a lot! If you're not dying and can afford one get it, but don't get one if your going to die right away with it.

This guide is very small with only some basic tips and knowledge and was made for a friend who needed a bit of guidance, if this helps anyone i'm glad to have helped you, this is a Support/AP guide for Sona and is only one of the ways you can play/ build her, please have fun with it. This is only an opinion of how to build her, you can do as you please with the build, its always wise ( and fun!) to experiment with it. Thankx for reading and enjoy!