Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
TL; DR: Skip to the end
Taric is an extremely effective support at all stages of the game. He has very high survivability, is an excellent choice for a secondary tank (poor choice as primary tank), can output frequent heals if left to auto-attack, is a great pusher, and complements a wide variety of team makeups. An otherwise 'bad' team can win if Taric does his job correctly.
-- Get Imbue first, as it's paramount that neither you nor your laning partner (or anyone else you come in contact with) dies. Actually, you want to make sure to save enough mana for a stun, but otherwise always keep life totals topped off.
-- Second skill is Dazzle, your ranged stun. It's better to wait until the opponent overextends, stun from a distance, run up and make them pay--then run away as stun wears off. Usually, you'll get a free hit in. Obviously, CC is amazing to have ASAP.
-- Next, you want to max Shatter. Shatter passively provides you and your laner with armor and is an amazing short-ranged AoE nuke. Shatter does a surprising amount of damage if leveled early, penetrates opponents' armor, and decreases it for your laner's convenience. Unfortunately, it temporarily removes the passive, so it should be used wisely.
-- Radiance should be taken at every opportunity, although you shouldn't use it much outside of skirmishes, one-or-two-whack harassing, and tower pushing. Radiance can be very mana consuming, but amazing for tower pushes and team fights.
Early-Game / Laning:
Buy a mana manipulator and run to your lane. If your teammate uses mana, great--he'll have a -lot- to use for farming or harassment. If not, it's still worth getting for Soul Shroud's cooldown reduction later.
At this point, Taric doesn't need anything to be useful. It's very unlikely you'll run out of mana with Manipulator and improved Clarity (use clarity for your laner even if you're fine on mana), you can heal yourself if you're low on mana, Shatter passively gives you an advantage in Armor, and your Magic Resist runes passively scale as enemy casters get stronger.
Once you've determined which enemy is squishier, try to pop Shatter and run away as often as health, mana, and zoning allow so that you can keep up the harassment (try to get a few last hits with it at the same time). Auto-attack to heal more often and if the opponent overextends, stun them and make them pay for it.
You don't need gold (you'll be here all day). You should find a happy medium between last hitting, harassing, and pushing, but ideally you help your laner get kills and last hits.
Ideally, you would hit level 12, bluepill, get Soul Shroud + boots, and head to where your teammates are to push towers. Even if you couldn't push your own lane's tower, you'll often find that your presence in any lane will be too much, and they will domino. It's not a bad idea to kill the Golems, Wraiths, and Wolves in the jungle after level 8 just so that you can get experience and gold independently of your teammates. Your next item should either be Guardian Angel or Aegis of the Legion; if you personally are getting bursted down, get GA, otherwise get Aegis for the team.
You have so much armor and magic resist at this point that there is diminishing returns on anything else that you buy. If you deem it necessary, by all means get Banshee's Veil. Nashor's Tooth allows you to be much more offensive and supportive simultaneously, and is a great item for Taric, but usually won't even be necessary. The sixth item is so situational and likely so expensive that I dedicate myself to buying wards, oracle's, or whatever other useful item the team needs.
Mana manipulator, stay in lane without recalling until you push your tower or are 12, keep everyone topped off and alive, and try to stay with as many people as possible once mid-game hits.
Enjoy, he can be outrageous.