What's the controls for the game?
There's a "Help" menu with general controls, a "Keys" section in the options menu where you can change controls, and in the tool palette (you can open this by pressing the tool icon in the bottom right, or pressing "T") there's "?" button with a brief description of how the tool is used and any extra controls.
You can also directly show the help of the current tool with the "H" key.