You've not said which Canon scanner you've got. An ordinary 'bog standard' flat bed scanner works by reflecting light off of the surface of a photo or document. Scanning negatives requires that light is transmitted through each negative. So it's not possible to scan negatives with a 'bog standard' flat bed scanner.
However, some Canon flat bed scanners have a built in negative/transparency adapter. They've also got a light source built into the lid of the scanner, so that light can be passed through the negatives. If you've got one of those scanners you simply insert a strip of negatives into the special holder, clip it into place and close the lid. Then, in Photoshop, go to File > Import > Canoscan 4200F (or whatever model of scanner you own) to open the TWAIN driver. Use the drop-down menu to set the source to 'Negative Film (Color)'.
Chris