Guide the hero through various levels and shoot down all the enemies, in this awesome sequel with more features. Use ARROW KEYS to move, CTRL KEY to throw a punch. Use YOUR MOUSE to aim and LEFT CLICK to shoot. To change your weapon pres "F" KEY.
There's a fairly in depth tutorial you should read to get you started.
Crackdown creates an exciting open world and gives you a lot of interesting abilities, but there isn't enough main-game content to truly take advantage of the game's strengths.