Just stack as much defense gear as you can get a hold of and you should be alright. I recommend tanking some of the Outlands instances to get used to holding aggro. The thing about the Northerend instances is that if you screw up, the party wipes. Read up on the bosses so you know what to expect, make sure you can hold aggro.
As for the safe level to tank the instances? That depends on how much gear you have and how much gear the healer has. If you have a really good healer you can tank it at lowest level, if not, tank it at the top of the levels for that instance. (Like if UK is 68-70, tank it at 70.) I didn't have spectacular gear when I went into Northerend and managed just fine, so keep getting that defense gear and you will be good to go. I would also recommend you make friends with a blacksmith, as they can get you some good tank gear to level through.