In short — click+drag to select things, and shift+click to do stuff. You can read detailed instructions by clicking the Tutorial button in-game and then playing the level or clicking “I’d Prefer to Read”.

Update June 07, 2012 — Added fast-forward and zoom. Press "/" to toggle fast-forward and the quote key to toggle the magnifying glass -- or click the buttons that appear at the bottom of the screen when no units are selected.