For running Heroics at level 80, you are going to want to shoot for 535 Defense.
For raid content, that defense number becomes 540. This makes it so the enemies can't land critical hits.
After that, you'll want to prioritize Stamina over Strength. Try to raise your health to a respectable amount. If you have rep with some different Northrend factions, you can pick up some armor pieces that have good defense rating from them, otherwise picking up the Daunting Legplates and Daunting Handguards are two good pieces to grab.
If you have some funding, the Tempered Titansteel Helmet and Shield will be great pieces to pick up once you hit 80.
Until you hit 80, the 540 Defense rating isn't really as vital, just grab whatever Defense gear you can (the Daunting pieces I mentioned can be picked up at 77 or 78). Maintaining threat on enemies will be your primary concern. To help keep threat, you should always be in Defensive Stance. Thunderclap and Shockwave are both good AoE threat tools, but especially for groups you'll find that those two often aren't enough. You will also want to use Cleave a lot, and make sure you switch through your targets and hit each one with Devastates or Shield Slams, while continuing to Cleave and drop Thunderclaps and Shockwaves. Also, since it stuns, remember to use Shockwave to help you position mobs. I usually Charge in, Thunderclap, back up a bit so they all group together in front of me, and then Shockwave to stun them all.
The key to this is going to be practice. Find a group that is willing to let you learn as you go, and is able to help you figure out tactics, etc. A group of helpful guildmates is always a good plan for gaining experience.