Start tanking immediately. Starting at the low levels will give you teh practice you need for a higher level.
I tried to start tanking with a Paladin, but it didn't really fit my playstyle. I really enjoy the warrior tank.
You might also want to consider waiting for Cata to come out, because if you are in mid leveling of the tank it might through your whole gears off.
You also might want to consider goign through and resetting your warrior, and practice by working runs in the Stockades or the Deadmines.