Find several basketball posters. They might have players or pictures of games on them. Verify they are small enough to fit inside the locker you are decorating. Cut them down to size if necessary. Locate a mini-basketball hoop that can be glued or stuck to a wall using a suction cup. Get a mini toy basketball and a couple of sports magazines that highlight the sport with full-page pictures. Most of these items can be bought at sports equipment stores, or on Internet auction sites.
Cut your favorite pictures out of the magazines. Neatly cut their edges so they are even. Tape these pictures and the posters so they cover the inside walls of the locker. You can cover the locker space entirely or leave bare spots if you prefer. You can also create a montage out of several different pictures of players, basketballs, basketball courts and games. Cut the pictures out and glue or tape them all together on a large sheet of paper, which you tape to the locker walls.
Attach the mini-basketball hoop to the inside top of the locker door with strong glue or little, rubber suction cups. If you want, you can close off the net's bottom and toss in a miniature basketball for added fun. You can also leave the net open so it can be used for shooting hoops with the mini basketball. Since the locker is made out of metal, you can also attach assorted basketball-themed magnets to the inside walls and around the mini hoop.
Attach basketball-themed stickers to the outside of the locker door. You want to use stickers for the outside because taped-up pictures and magnets may be stolen or torn away by other students as they pass. Arrange the stickers in an artful pattern. For example, place them along the edges of the door with a larger sticker in the middle.