You need
* a stereo 'sound in' socket on your PC's sound card. You'll only get a mono recording if you use the 'mike in' socket.
* a lead to attach the tape player's output to the PC's port
* software to record the input (possibly already on your PC otherwise plenty of free alternatives you can download.
* software to convert the recorded .wav file into an .mp3 or .ogg compressed audio file if you wish. Some software can both record & compress the audio input in one step.