There are several ways to achieve what youre looking for. The first is to adjust the lightness and the contrast in photoshop to see if you can make the white paper as white as possile without losing some of the detail in your sketches. Another quick and easy method is to use the pen tool to select your image and then you can copy and paste it onto a new file with a uber white background. Others like adjusting or actually applying the multiply “filter” can eliminate the white parts, but may affect your actualy sketch. Just play around with the brightness and contrast to see if that will work. other than that cut and paste is your friend