The program may give you the dimensions of the resulting scan, which is helpful if you are trying to create something to fit a specific size. Select the area you want to actually scan on the prescan image. Typically there is a marquee tool to select a rectangle on the prescan image, and you can adjust the edges, and move the entire rectangle around the scanning area. If the program you are scanning with allows it, you may choose to make adjustments in various exposure, contrast, and color controls at this time. The quality of your original will directly affect the quality of the digital image you can create, so choose your originals carefully.
Cheap reproductions are usually troublesome to scan.
Before you scan, think ahead to what the end use of the image will be. If the image is for a PowerPoint presentation or the web, a computer screen only displays 72 dpi, so any more is really a waste of disk space. File storage space is not unlimited. While it may seem like a great idea to scan images at the highest resolution possible for the most detail, it is very important to keep your graphics files lean. 300 dpi images of even small dimensions can take up megabytes of disk space.