You can boil shrimp with the shells on or shrimp that are pre-cleaned. Many people, including me, believe shrimp boiled in the shell has more flavor. Either way they should be totally thawed, either in the refrigerator overnight or under cold running water. Use the running cold water method only if you are in a hurry. Shrimp that is thawed slowly and rinsed very briefly tends to keep more of it’s flavor. Remove the dark vein on the back before you boil shrimp.
Have a pan large enough to hold the shrimp with about 2 inches of water above the shrimp.
Add the water to the pan and season it if you like. Plain salt or something like Old Bay Seasoning will work fine.
Bring the water to a rapid boil. Turn the water down to a simmer and add the shrimp.
Boil shrimp for 1 -5 minutes, depending on the size. When a couple of them float to the top they will be done. The thickest part of the shrimp should be opaque white.
Immediately put them in ice water to stop the cooking, and drain in a colander. They turn to rubber if over cooked. They will curl up when cooked and will look like the letter C when they are done. If they look like the letter O they will be over-done.
If serving hot skip the ice water. Shrimp get rubbery when over cooked.
For information on other types of cooking shrimp clink on the following links.