Crabs are sold by size: small, medium, large, jumbo and occasionally colossal. They can be very expensive (especially if you buy them already steamed) and the price difference between the sizes can be significant. I recommend getting at least the medium size. The size difference between medium and large is often slight but the price difference is frequently about $10 a dozen.
Picking crabs is time consuming. Unless you are a professional, plan to spend the bulk of an afternoon or evening picking crabs.
Your hands will get very dirty. I find it is difficult to eat anything but crabs while you are picking unless you're willing to get up and wash your hands frequently. If you want to serve other food, I suggest serving it before or after the crabs.
Avoid touching your eyes while picking, seafood seasoning burns. Also, it is not uncommon to cut your finger on a shell but it is generally no worse than a papercut.
Cover a large dining surface with a thicker than you would think layer of newspaper. It really is best to do this outside on a picnic table if at all possible. If not, resign yourself to cleaning the floor afterwards.
Pick out a crab. Try and get one that has both claws and feels "heavy" before someone else does.
Bend or twist the legs and claws to snap them off at the body. Some times a bit of crabmeat will come off with the legs or claw. Eat it. Set the claws aside. There is not much meat (if any) in the legs so I just push them aside.
Pull off the "apron". Some people use a butter knife for this but I am a purist and only use my hands. Simply slip your finger under the edge of the point and pull down. It should pull off easily.
Pry the shell away from the body. The easiest way is to use both hands and pull the crab halves in opposite directions.
Flip the crab over.
Remove the squishy, grey gills. They pull off very easily. Discard them. The yellow stuff is colloquially known as the "mustard" and is edible.
Crack the crab in two. Pull out any loose crabmeat and eat it.
Crack the halves and extract the meat. Again, some people use butter or crab knives but I find that fingers work just fine. Eat the meat as you go.
Hold both sides of the crab claw and break off the part that I am holding in my fingers.
The meat should come off on the claw. If not, break the claw with your mallet.
The other half of the claw has meat as well. Break it off at the joint. If this doesn't yield meat, hit it with your mallet.
Repeat, repeat, repeat.