If you've never played this game before, you will want to choose the Easy option. The reason you'll want to begin with this option is it's the best way to get a feel for the game without driving yourself crazy.
You will use your mouse to select pieces and drag them around the screen. You'll notice that many of the pieces are not turned the right way. If you need to rotate a piece, press your spacebar once the piece is selected. Each spacebar click will turn the piece once.
A timer at the bottom of the screen keeps track of how long it takes you to complete the puzzle. While mastering the Easy level is great, it's far from being the end of this game. There are four more levels of increasingly difficult solving you'll need to master in order to conquer this game.