Firstly, congratulations on level 70. Now the fun begins!
Really you need to have a look through the guides on pre raiding gear and work towards that prior to thinking about enchants I would suggest. However you do have two choices enchant wise, if your rich then you can enchant everything, even small upgrades. If your not, you need to work towards a solid set of gear and get that enchanted asap with the right enchants.
Defense on chest, cloak and bracers are good choices, although stamina on bracers is also very good. The Keepers of Time head enchant [revered rep] is a must have. Shoulder enchant from your faction [aldor or scryer] is another must have. Gloves as soon as you get a good pair should have +2% threat on them ideally, but +15 agility is another good choice dependant on requirement. Your shield, if struggling for uncrushability should have +15 Block rating, this can be upgraded to +18 stamina at a later date. Clefthoof leg armour is good and when you get a good pair of tanking legs then Nethercleft is your best option.
All of these enchants are covered in the sticky enchanting gear guide by Invis

I would suggest reading it if you havent already, it really helps.
As for Tanking a heroic. No, you are nowhere near ready, you dont have the health, avoidance or spell damage to cope with it at the moment. Off Tanking Kara? You might be lucky and get a few good drops of the first couple of bosses, but I wouldn't take you in in a group I ran in honesty at the moment, you aren't ready gear wise.
You just need to get the upgrades one at a time now, if you have a supportive guild then they might fast track you to get some gear, failing that you need to get the upgrades you can in small steps and soon you will be a viable choice for easier heroics and Karazhan.
I don't mean to sound harsh, but you have a way to go, getting to 70 was the easy part

You will succeed, just dont try to run before you can walk as there are thousands of trigger happy DPS in BG Arena gear who wont give you a second chance. Rely on your guild for support and ask for help whenever you need it, either from your guild or on these forums.
Good luck Tankadin.
Barathorn
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'Don't mind Guillex, he thinks he's Dorvan' - Lore at his best March 2008.
http://armorylite.com/eu/Aerie+Peak/Barathorn