To be a tank, you need a horse. If you just want a heavily armored shielder, you are a turtle, not a tank. :) Still a fun and worthy build though.
First of all, use this character planner to practice making builds:
http://infinitum.dyndns.org/crpg/calc.htm
My suggestions:
Turtle:
Level 30 (4 420 577 xp)
* Strength: 21
* Agility: 15
* Hit points: 70
* Skills to attributes: 2
* Ironflesh: 7
* Power Strike: 7
* Shield: 5
* Athletics: 5
* Riding: 0
* Horse Archery: 0
* Power Draw: 0
* Power Throw: 0
* Weapon Master: 5
* One Handed wpf: 146
Tank:
Level 30 (4 420 577 xp)
* Strength: 21
* Agility: 15
* Hit points: 64
* Skills to attributes: 2
* Ironflesh: 4
* Power Strike: 7
* Shield: 4
* Athletics: 5
* Riding: 5
* Horse Archery: 0
* Power Draw: 0
* Power Throw: 0
* Weapon Master: 4
* One Handed wpf: 138
With either one, equipment is an important part of the build. For a shielder/turtle, you'll want heavier armor and a tough shield. For the tank/cav you'll want heavier armor and a heavy horse. Weapon is personal preference.
Also, there are lots of ways to make builds like this! Try out that character planner and listen to other people's advice as well.