Read along with the Prologue and try the examples they give, especially the ones with the rocket. Pay special attention to the sidebars. The easiest way to get everything working is to add the following lines at the top of your file after doing Language→Choose Language and then clicking the top radio button, which says The Racket Language:
#lang htdp/bsl (require 2htdp/image) (require 2htdp/universe)