Leeks need trimming. Cut off the bottom of the leek, just above the root hairs. Cut off the top of the leek, just below the "V" shaped base of the dark green leaves. Slice the remaining white/pale green shaft of leek down the center, in preparation for the next step.
Wash the leek. Make sure the water flows in between the many layers of leek. Those nooks and crannies can hold a lot of sand.
And now you're good to go!
Check out these sites for tasty leek recipe ideas:
EatingWell has everything from apple and leek stuffed pork to leek, potato, and spinach stew.
Bon Appetit has sophisticated dishes like goat cheese, leek, and potato galettes.
Saveur has twists on classic leek dishes, like cream of fennel soup with leeks.