I expect you placed a complete letter into your scanner and thus you have scanned the letter head at the top, but all the white paper underneath. Then, I think you may have added the scan as an IMAGE that fills up the whole of the page in Word. You are then trying to place text onto a white image, rather than Word itself.
I think I would take the scanned image and edit it in a picture editor (Paint Shop Pro perhaps) and get just the bit at the top. Now place this much smaller image in Word, and type underneath.
Note that Word is a pain when trying to place images, you may have to try various settings before you get it as you want.