Scratch

Make a variable: scrollx is what I'll call it, and make your terrain out out of sprites. Make the first sprite of the terrain have this script: forever set x to scrollx + 480 times 0. Make your second terrain sprite have the same script exept the times 0 at the end should be made into times 1, the third terrain sprite has times two, and so on. Make the right arrow change scrollx by 1 and the left arrow change scrollx by -1. That's a scrolling program.