Depends on your OS.
For Windows, make sure you have installed the Canon software/drivers. After that, go to Properties for the printer (how you get there depends on OS as it can vary depending on whether you have XP, Vista, or Win7). Once there, you should be presented with a window that has 7 or so tabs. Go to the Maintenance tab. On the Maintenance tab there should be an assortment of buttons, one should be entitled Print Head Alignment. Other buttons may vary with functionality of your Canon device (i.e. you have a multi-function printer/scanner which may have different options available over just a straight up printer).