Basically, you're going to saving the AD carry. That simple. You can play offensive, while doing a good job supporting, and keeping the enemies at bay, with your silence.

The runes provide you with more gold, without giving up much defense. You are playing Soraka after all, one of the most squishy champions in the game. The magic resistance per level is quite good, as it gives you more in the long run, and you should only have to worry about AP carries in the long run. The armour is for their AD carry.

The masteries are again, for that bonus gold, and so that you aren't that squishy, like some people; Vayne. The masteries are allowing for maximum income, and for health, the armour choice is better, as you are in lane with an AD carry. You do not have to use Sage, but when I support, the AD carry usually kills a lot, so I get more EXP.

The items are again, allowing for the most income, while making you tanky, and mobile.The silence will proc your Rylai's slow, allowing for you to keep up a chase, and to slow for your teammates. I get the Morello's Evil Tome over Deathfire Grasp, because it allows for more mana to be used in bursts, and has a higher cooldown reduction effect.Rabadob's comes last, because I go for the more tanky approach before, and your heals are strong enough without it. Shurleya's Reveire is just for the movement speed increase, and to give you some sustain mixed with cooldown reduction. You start with 4 wards, to assist your carry, and to give vision around. If you are purple team, walk around to the river side, but on the side with the tri-bush beside your tower, and ward over the wall, close to dragon WITHOUT going into the river. IF you're not, you have to walk over there, but no big deal, just be careful.

The reason I get Starfall as my last skill is because it doesn't do that much damage and burns mana, and you want to level up your mana and heal restores as best as you can, to provide your AD carry with maximum slapping power. Your ultimate is obviously going to be leveled up first, as it heals EVERYONE for more.

The reason I use teleport, is simply because I cannot count how many saves I have had using it. It allows you to stop pushes, and teleport to teammates to save them from tower dives, or just to simply hold their lane if your jungler is pre-occupied, dead, or top is just over whelmed. Flash is for those pesky runners, to slow with Rylai's or get the last starfall or anything like that, quick saves on teammates, and just for escaping personally.

All and all, you can sustain, save, damage, and you're very mobile. All in all, you're a perfect support, and a better champion. Even though your cooldowns are high, I am sure you will make due with them, and know when and when not to use spells. You can be aggresive, but it is not that advisable unless you are sure you will not die, or are comfortable with your skills.