1. Don't take farm when the AD carry or any other gold-needing friend is able to take those farms. (If your friend is somewhere else, feel free to take the last-hits.)

2. Don't take kills when the AD carry or any other gold-needing friend is able to take those kills. (If your friends can't take the kills, make sure to last hit them.)

3. Your strenght lies in the fact that you boost your allies. In some cases (sona, blitzcrank, lulu for example) can inflict nice damage. Yet remember that you are a support, you get defensive items and max defensive spells.

4. Nearly all botlane kills (laning phase) are set-up by supports, this means that you are responsible for the ganks. The jungler doesn't start a gank, YOU do. YOU ask for the ganks, time their wards (will get back on that later) YOU will tell the jungler (ping) where they warded and where the jungler should go.

5. You are a map-control-tower. You make sure that the map is warded, you use your CV to protect allied lanes from ganks (Ill get back on the use of CV later) and you are the one timing all stuff (again, will get back on that later.)

Sona, she's been the best support for a while, she just got thrown of the throne by janna (In my humble opinion). She delivers decent sustain, decent damage and a huge load of CC.

I will not spend time on writing down all the moves and their effects, I sugest you read the descriptions when you move your mouse over the ability instead.

Summoner spells
Flash: Sona uses flash to escape from ganks and to get in range for a game changing ult. Also, I've found exhaust rather useless for it's slowing purpose on Sona's lanes.
CV: For the use of CV, I redirect you to the section where I explain my chose for this awesome spell.

Items
[STEP 1] Fearie Charm, Wards, Health pots
Cheapest item ingame, combined with the essential map-awareness and an hp.
[STEP 2] Philosopher's stone
You need this in order to maintain your sustain and to obtain the gold you need for your build, since you don't get farm/kills.
[STEP 3] Movement Speed Boots
Obviously, must-have.
[STEP 4] Heart of Gold
More gold/sec for the build and some more health, makes you less squishy.
[STEP 5] Boots of Lucidity
Make you movable, anyone needs finished boots, also the cd reduction works on your Crescendo.
[STEP 6] Aegis OTL
At this point, you can make your carry a bruiser with carry damage, aegis combined with the W aura, makes you tanky as hell, and with you, your carry.
[STEP 7] Shurelia's
Pop it, your team is faster, great for chasing and fleeing. Also it gives a lot of mana/5 and health.
[STEP 8] Locket of the Iron Solari
Underrated item in my opinion, it shields your whole team for some damage and gives them enough health/5 to siege towers.
[STEP 9] Zeke's herald
Makes your AD carry a machinegun with self-sustain, it is a great buffer for your team and it gives some health for you.
[STEP 10] Abyssal Scepter
Increases your AP and MR and at the same time decreases the enemies MR.

Ofcourse, this build can be changed depending on the game, in a double AP team for instance, you can always go WOTA instead of Zeke's. Also, when they have a great focus team and no AoE, the locket of the iron solari won't be as effective as it is otherwise. Change these items a bit depending on the game, keep in mind that you're a support and stick to my advice when you're not sure.

Taric's just gay, and therefor he's op... Kidding, Taric is a fine support, healing is decent in pre and midgame... The stun is a nice thing to pressure your enemies with. And besides, taric's passive allows him to use his ult and his heal more often, it's a nice thingy. Taric is great for boosting your team, he gives them a lot of armor, attack damage and ability power!

Summoner spells
Flash: Use flash to get in range for a stun, use flash to get right in the middle of a teamfight, there, where you can boost ALL the allies.
CV: For the use of CV, I redirect you to the section where I explain my chose for this awesome spell.