Some scanners do not scan all the way over to the sides of the glass, but leave a small gap between the edge and the scan head. One solution for this problem is to place a straightedge on the scanner glass. Place it away from the rule printed outside the glass, if the scan head does not extend all the way to the side. I've used a ruler or card as a straightedge, effectively extending the scanner's rule out over the glass to the picture edge. It's best to avoid having to straighten an image, because this requires rotating the image.
Rotation is one of the most difficult tasks for photo editing software.
You will need high quality photo editing software to prevent the image from getting fuzzy or being distorted. Correct the Image Use the various tools in your image editing software to make any corrections to the image. Some of these tasks, such as removing a moir pattern may work better if done during the scanning process. Often scanning software will offer corrective features during or right after the scan. Unless you're looking for some special effect.