Like all programs, this program starts off with an event. Usually this event is when the user clicks the green flag. So go ahead, smack dat flag!
I made the last backdrop. Credit goes to Scratch for the sprites and other backdrops! This will talk about what you can code in Scratch. You can always see other projects that will give more detailed explanations of practically everything you can do on Scratch. Here's the Scratch wiki if you need help: http://wiki.scratch.mit.edu/wiki/Scratch_Wiki_Home