I believe a step-up is when there is a jump at the bottom of a big dirt mound, or hill. So instead of riding up a hill, you jump up it.
Listen to Gomers advice on jumps, it's good. I wish I had that advice when I first started jumping. I built a small wooden jump that was short and had a steep incline. It made it incredibly hard to learn to jump from that. I would have to say that a good begginner small jump would be about 5 feet long, and maybe 2 feet high. And be sure to keep a steady throttle when going off, any sudden acceleration or deceleration while on the face of the jump could cause you to crash.