Connect a walkman or stereo to your computer using a 3.5mm audio cable which can be bought for a few quid. It's just a thin cable with a 3.5mm jack at both ends. One end needs to be plugged into your walkman/stereo's headphone socket and the other end connects to the PC's headphone socket on the back of the computer.
Then use an Audio program such as Audacity to record the tape's contents onto your PC in WAV or MP3 format depending on your program's capabilities. I have the settings for this procedure written down somewhere - I'll look for them.