Massive Nerd

(For a full listing of Character Guides, please go here)

Clank.

Unless you're using Shale in his Stoneheart form, you'll need a Warrior for tanking.

Party Members

Alistair is ready to go in this role.
Sten starts with a couple points spent in Two-Handed talents, but could be switched to tanking with minimal loss.
Oghren and Spoiler join your group too late in the game to be effective tanks, with several points already spent in Two-Handed talents.

Stats

Strength: 42 - This is the requirement for wearing the heaviest armor in the game. Anything beyond this is wasted, since it will only increase your damage, which is less important than your defensive abilities.
Dexterity: Maximum - Dexterity increases your chance to dodge attacks, as well as the amount of damage absorbed by armor. Every extra point you have should go here.
Willpower: 20-25 - Honestly, whatever you're comfortable with. Tanks use several sustained abilities, which will eat up your Stamina pool. The last thing you want is to be unable to use Taunt in an important situation. Your choice of Specialization will also factor in here.
Magic: Minimum - Nearly useless
Cunning: 16 - This is the minimum amount required for max-rank Coercion (for your main character) and Combat Tactics (for your companions).
Constitution: 20-25 - Again, whatever you're comfortable with, as well as adjusted for difficulty. Though, keep in mind, you only gain 5 HP per point.

Specializations

Templar: Alistair comes pre-loaded with this, so many think it's the Tanking Specialization. In reality, it isn't that useful. All of a Templar's skills are for use against Mages, which is a job better suited for your Rogues.
Berserker: Berserker talents are offensive-oriented. While not bad, your Stamina is better used elsewhere.
Champion: THIS is the Tanking Specialization. War Cry decreases enemies' chance to hit, which is very handy for a tank. Superiority gives War Cry a knockdown effect, and makes it invaluable. Rally & Motivate are great, but only if you invest in extra Willpower to offset the upkeep cost.
Reaver: Reaver is a decent SECONDARY Specialization, because of only one skill. Frightening Appearance increases the effectiveness of Threaten and Taunt, two vital Tanking abilities.

Talents

Instead of listing EVERY talent, I will just point out the important ones, both good and bad.

Powerful, Threaten and Taunt: Of course.
Bravery: Grants several offensive and defensive bonuses when engaging more than two enemies. This will be often.
Death Blow: While a Stamina return seems like a good idea, your tank will rarely be getting the killing blow.
Precise Striking: You have to get this as a prerequisite for Taunt. Additionally, it's a decent ability to use early in the game when you don't have a full compliment of abilities to drain your Stamina.
Perfect Striking: While this seems like a great ability to generate aggro, the Stamina cost is not worth it.
Shield Bash, Shield Pummel, Overpower and Assault: Nice aggro-building skills, and the stuns and knockdowns are very handy. However, use them sparingly to conserve Stamina and focus on your defensive Talents first.
Shield Defense and Shield Wall: Should be your top priorities. They do not stack, so Shield Defense should be cast aside once you get Shield Wall.
Shield Cover: Useful early on, but unnecessary once you have Shield Wall and better gear.
Additionally, all Passives in the Weapon & Shield tree are mandatory.