Well, obviously you'll need a tape player and connect it up to the computer. If the player has an audio out use that, if not use the headphone socket. Connect it to the sound in or microphone input into your computer.
The easiest connection would be audio out to audio in but you may not be this luck. Any other connection and you have to adjust the volume.
Does your tape player have any kind of output socket? If so, you'll need to run a wire from there to the input bit of your computer's soundcard : the soundcard will be the bit at the back where your speakers currently plug into. Next to that will be a socket (possibly lime green) with a wee picture of a microphone in it.
If you can connect your tape player to that then you'll be able to play the tape through your PC and record that sound.
One free program called Audacity will record any sound that goes through your PC like this.