There are other guides that are ornate in how they explain a powerful build for a character. This is not one. Simply, Taric is the best pusher/tank/support I've yet played in the game, and some recent games have confirmed my theories of how to build and play him.
Taric, the Gem Knight
Role - Tank/Push/Support
Phase I - Abilities
This should be your second priority to improve when leveling.
Imbue - This is your heal. Unlike Paladins from WarCraft III, however, if you use Imbue to heal another target, Taric gets healed as well. If you focus it on Taric, you gain 140% the heal's effect. Imbue scales with rougly 50% of Taric's Ability Power. Every time you strike with your melee attack, the cooldown on this gets reduced by a small amount, which has led some players to value +Attack Rate when playing Taric.
This should be your main priority to improve when leveling.
Shatter - This is your AoE and defensive ability. Shatter gives a self-buff that, while up, improve's Taric's armor. If you use the Shatter ability, it deals damage in a small radius and reduces the armor of whatever it strikes. Shatter scales with rougly 25% of Taric's Ability Power.
This should be your last priority when leveling, but should be taken early.
Dazzle - This is your ranged spell and stun. Dazzle shoots a beam from Taric's shield which has two effects. The closer an enemy is, they take more damage but suffer a shorter stun. Conversely, the farther away an enemy is, they have a longer stun but take less damage. Dazzle scales with rougly 50% of Taric's Ability Power.
This should replace Shatter as your priority beginning at Lv.6.
Radiance - This is Taric's Ultimate ability. Taric gains a 60% damage bonus, and surrounding entities/heroes gain a 40% damage bonus. Also, Taric and surrounding entities gain health while the effect is up. This drains mana over time rather rapidly. The HP restoration benefit is not known to scale with Taric's Ability Power.
Subsection: Useful Summoner Skills
Taric does this well, and pushes. If you push, you should have heal as a matter of principle, decorum, and pride. Pushers love Heal.
This is a way to overcome Taric's terrible movement speed (addressed in the Itemization section.) Also, no one expects to be following a wave of minions to a 'undefended' tower, when Taric pops up literally out of nowhere and clears the entire minion wave out from under them.
Phase II - Itemization
The basic build I suggest is:
Sorcerer's Shoes -> Chalice of Harmony -> Stinger (later, Nashor's Tooth)
Sorcerer's Shoes - LoL is a PvP game. You live or die, win or lose by maneuverability. Taric is notorious for his lack of speed. Also, some of his abilities do scale with AP to an extent. I suggest getting these by about Lv.3 to 4 for maximum effect. If you're going to be forced into a tanking role, you may consider switching these out for Ninja Tabi or Mercury Treads - other items, like Stinger/Nashor give enough AP that you can forgo the bonus from the Shoes.
Chalice of Harmony - Taric's skills consume a great deal of MP. Chalice of Harmony has a great passive benefit: you gain 1% of each 1% of your max MP you're missing (so, if you have 2000 max MP, and you're missing all of it, you'll regen 2 additional Mana that five seconds.) Chalice makes Radiance not only viable and helpful, but deadly. Chalice is a must-have for Taric. (Thanks to Crazy Demon for a clarification on this item!)
Stinger, upgrades to Nashor's Tooth - Stinger/Nashor addresses Taric's speed weakness yet again, but in two ways - attack speed (which is painfully slower, worse than his runspeed...), and Ability Cooldowns, which are also fairly slow. Stinger/Nashor also give some MP5/Ability Power, which for Taric is always a pleasant thought, if not your primary concern. Stinger/Nashor's is commonly agreed to be tailor-made for Taric.
Some helpful items I've found along the way:
Doran's Ring - Health, MP5, Ability Power.
Spirit Visage - HP5, still faster cooldowns.
Emblem of Valor (upgrades to Stark's Fervor) - An HP5 Aura (with Radiance, yields !!!), and lifestealing. Stark's Fervor gives an Aura that improves the DPS of nearby teammates, as well.
Aegis of the Legion - Magic Resistance, Armor, and Health. Futher, the Armor and Resistance are also given by an Aura. (Special Thanks to Bloodba7h for this suggestion)
Tiamat - Your melee attack becomes an AoE, you get some attack damage, and HP5 and MP5.
Phase III - You Like to Push It Push It.
(AKA: 4 simple tips to playing Taric effectively.)
Tip #1 - Taric is a great minion tanker. Just wade in the middle, let off a shatter, then run off. Let hilarity, and XP/Gold, ensue. Exploit this every opportunity you get. It's not uncommon for Taric to outlevel teammates by one to two levels if you love ganking minion waves and outthinking enemy players.
Tip #2 - If you are partnered with a buddy, make sure you heal him - you're healing yourself too. If you're in Summoner's Rift in a full-fledged 5v5, there is no reason not to be healing, while causing havoc because of the simple fact that you wipe minions off the table as a threat. Besides, healing your buddy heals you too.
Tip #3 - Be smart with Dazzle! Sure it's great for a bit of extra damage, but it's most useful because of the stun. If your ganker buddy is hiding in the bush, get low on health, lure them to the bushes...let off Dazzle...let buddy go to work. You just got an assist.
Tip #4 - Do not make an active push until Lv.6+. Radiance is one of the very best pushing tools in the entire game. Besides, below Lv.6 towers will make short work of you anyways. AND you have better things to do, like break up minion waves and get that Chalice.
This guide is no substitute for real practice with a character. However, think about these ideas, try them out, and define your own play style. Sure, in my admittedly short experience these principles work well, however any skilled LoL player will tell you every time: how you build your character depends on the overall game. I hope this proves a informative guide, and wish all Taric users well.