As Support: First go watch that youtube video about your area of influence.
Anyone have a link? Searching "League of Legends support area of influence" isn't turning up anything useful, and just "League of Legends support guide" is turning up a few too many things. --- http://i39.tinypic.com/xnar69.gif
Remember that you can do things as support even without using your abilities. If you're leona you can use the bushes as a way to zone the enemy carry. Hide in bush and pop out when he comes near. You can make them scared of getting CS or you can catch them as they go in for CS. Same thing goes for Taric. The enemy knows that you have a targeted stun and high burst. You can force them out of farming range with the threat of a stun.
There's more nuance of course. Knowing if your adc has the health/damage to win a trade if you make a stun. Figuring out how to dodge enemy skill shots or knowing when to eat an ashe volley so that your carry won't be hit by it later etc. --- "SR'ing is having the RNG abuse you." Deshokun
Where to learn good AD: Watch Aphromoo play Draven Where to learn good Support: Watch Nhat or Aphromoo play support
also, always play Leona support. If someone else picks another support, still pick Leona. Your radiance will blind the bad supports into submission, allowing you to control your lane with grave and elegance.