Deveining the shrimp is a personal choice rather than a necessity. The black sand vein line that runs down the back of shrimp is its intestinal tract. In small shrimp it's not really noticeable, but in large shrimp the vein is unappealing and can add a gritty, muddy taste. There's no harm in eating shrimp that aren't deveined, but if you prefer to remove it, you can use a special deveining tool or a knife. Slit the shrimp lengthwise down its back, and pull away the vein with the tip of the knife.
If a recipe calls for shrimp and you want to start with unpeeled fresh shrimp, you'll need 2 pounds in order to end up with 1 1/2 pounds of peeled shrimp.