Before pressing the scan button, you need to first press the preview button (sometimes they are called different things like preview, prescan or overview but the button is usually to the left of the scan button). Once you do this initial scan, the photo should be visible. At this point, move your cursor over the photo and draw a box around the part of the photo you want to have scanned. This could be the entire rectangle of the photo as you see it, or if you want to focus in on something in particular and cut out the background, just draw the rectangle only around the portion you desire. The rectangle is usually a dotted line. If you need to adjust the rectangle, place the cursor near the middle of whichever side you need to adjust and the cursor will turn into a little arrow, allowing you to move just that one side.
As someone mentioned earlier, the DPI (resolution) of the photo should be set to 300 dpi if you will be printing the photo or 72 dpi if you only intend on viewing it on your computer monitor.
When the rectangle is where you want it and you have set the DPI, now you are ready to press the scan button. It will only scan the area you have put the rectangle around, and won't show any of the white area around it.
Hope that helps!
-Laura