Many ways to do it ...
Obviously you need a tape player
You need to connect the output of the tape player to the PC
Your PC should have a "line in" jack .. it's like a normal stereo jack ... all you have to do is get a cable with both ends looking like a headphone stereo jack.
I would say almost all PCs would have a line in. If yours doesn't, you might be able to use the MICrophone connector ... but in that case, you may only get mono instead of stereo.
Now you need a recording program ... Sasun pointed you to one. There are many others. Windows comes with a limited sound recorder that can record to WAV format. Files will get BIG really fast. You then need to compress them to MP3.
My CD burning software (Nero by ahead software which I highly recommend) has very nice utilities to do this. I think other CD burning packages come with some form of either sound recorder to mp3 converter.
Hope this helps get you started. Your PC sound mixer should allow you to set the volume (or unmute) the line in. Make sure you check that if you have trouble listening to the audio coming from the tape player on the PC.