if you're talking about an ordinary printed photo, you'll need to scan it on a scanner, which converts the image to digital pixels. Exactly how it gets from there to the PC will depend on the software in both, but typically the scanner is connected to the PC and made available instantly as a digital file.
Or if you don't want to invest in a scanner and do the hard work yourself, I believe shops like Jessops will happily put your pictures onto a CD for you if you bring them in. They certainly used to, so assume they still do now.