If you really want to fill this role, you must realize you will have to build two full sets of gear. You will need your tank set and your cat DPS set. You may also be required to build other sets such as Fire Resistance gear.
For the tank set, the essential items you should plan on getting are:
Bone Ring Helm
Atal'ai Spaulders of the Monkey (any +stam is okay, but Monkey is the best)
Sergeant's Cloak
Warden Staff (or possibly Unyielding Maul)
Slaghide Gauntlets of the Monkey (random stat roll like the shoulders)
Ring of Protection
Thrall's Resolve (Horde only; not sure about the 2nd ring for Alliance toons)
Mark of Tyranny
Smoking Heart of the Mountain
You will have to level enchanting to 265 to get Smoking Heart of the Mountain, but you can drop enchanting afterwards. Both of the trinkets never get replaced. Both of the rings are currently best in slot; Ring of Protection eventually gets nerfed to 150 Armor and becomes just decent instead of amazing.
For cat DPS, most of the current best in slot is identical to the rogue list. So I would suggest looking up the appropriate thread on Nost forums. There are a couple pieces that are different for feral but the rogue list is a good start. Obviously, you want Strength enchants over Agility since 1 Strength = 2.4 AP in cat form.
Lastly, you need to learn how to tank effectively (threat, positioning, what you can do in a high pressure situation, etc.) as well as how to optimize cat DPS. For that, the most difficult thing to learn is power shifting to gain energy from Furor without dropping any ticks. Other than that, the rotation is Shred to 5 Combo Points and Ferocious Bite as finisher. Keep Faerie Fire up on everything unless your raid leader says otherwise.
This is the spec I use for raiding:
http://db.vanillagaming.org/?talent#0zLVhoZg0xsfbdtV