Box jumps can be performed on a box, small ledge, high curb, park bench, stadium bleachers, or just about any stable platform you find at a comfortable height.
I like to do box jumps at the park on a stone wall that comes up about waist high, but typically box jumps are performed on 2 foot high platforms. Make sure you stand up completely on top of the box, fully opening your hips. You can jump or step down after jumping up, and if you get tired you can step up instead of jumping up. Just make sure that if you are stepping, you alternate legs.