The quality of your scanner and the size of the original photo will be the limiting factors to getting a decent picture.

Easy button is to take it to a local place that does printing services and ask them to scan it in for you.

If you want to mess around with it more then you need to study DPI or resolution settings on your scanner and set them to max before scanning. Then crop the image in a photo editing program. Then adjust size.

Easy for someone who knows how and has a decent enough scanner but hard to explain over text.