Your locker is sort of like your bedroom. You can customize it any way you like. Here are some tips you can use for making your locker your own.
Step 1: Locker Wallpaper
Using actual wallpaper is not the greatest idea. Use something like wrapping paper. Obviously wrapping paper is found pretty much everywhere, so thats pretty much the easiest alternative to wallpaper. During study hall or at the beginning of the day, measure your locker. Then measure out the amount of wallpaper you need, then use magnets to attach it to your locker door.