First: scan your original
These days you don’t need an expensive scanner to get remarkably good results when scanning photographs. My cheap CanoScan 670N does an excellent job. Whatever scanner you have will have come with the necessary software to interface with Photoshop. You access the scanning process via File > Import > . You should see a drop down with available scanners listed.
When you select the scanner, it should run the proprietary software that came with it. This will allow you to set the scan resolution, color mode etc. The interface varies from scanner to scanner. Suffice to say you need to select the area of the photograph you want to scan, the resolution and color. I'd suggest you start with 300dpi and full color. You can always change things in Photoshop later.
Tip: if the photograph is small like the one I'm using in this exercise, scan at least at 300dpi or even 600dpi. The chances are you'll want to crop the image and print it out at a larger scale than the original so there's no sense in starting with something that's too small.
For this exercise, I've provided a scanned image to work with, called original.psd. It's the output of a Canon scanner in color at 300dpi. Open it in Photoshop and we'll begin.