If you're 85 and have a character that you're thinking about playing his tank spec with, I would start with normal level 85 dungeons. Actually, you might tank Northrend dungeons for friends; it will be an easier way for you to get your feet wet without being too difficult.

If you're about to roll a character you want to tank with, start tanking right away. You'll get instant queues and you'll have a great head start on how to tank.

Actually...before you tank anything, read up on tanking concepts, and techniques for the class you choose. There is no difference between the various tank classes that would make one "the best", otherwise Blizzard would just do away with the other three altogether. Each class has advantages: paladins can bubble themselves and heal in a pinch, can multi-target pull, long range stun, and AoE tank with consecrate, but are highly vulnerable to silences; warriors are probably the most mobile, have a long range silence, lots of short range stuns/interrupts/silences but have limited self healing, can AoE tank well with Rend and Thunder Clap, and only their shouts and Thunder Clap are vulnerable to silences; death knights are better than warriors at self healing, can AoE tank with Death and Decay and Blood Boil, but their diseases (probably, I'm not sure) are susceptible to magic resistance and silences, and since they don't use shields they need high parry; druids have very high dodge and health because they wear leather and don't use a shield, can rejuvenate themselves, but are the weakest for AoE tanking.

I'm sure other readers will correct me on those and other fine points (I have limited knowledge of DK and druid tanking), but the main point is that each class has its strengths and weaknesses, but all four classes are perfectly capable of tanking. It's moreso the skill of the player in holding threat and self-preservation that determines how good that tank is.