Check the inputs on your cassette deck to determine what kind of stereo RCA cable you will need. If you see outputs labeled "audio output" with red and white inputs, you will need an RCA cable with the red and white plugs. If you don't see these outputs, use a mini-jack stereo cable that plugs into the headphone jack of your player. The other end must be a mini-jack connector.
Plug the red and white cables into the audio output on your player, or if it doesn't have one, plug one end of the mini-jack stereo cable into the headphone jack of your cassette deck.
Plug the other end of the stereo RCA or mini-jack cable into the line-in jack of your PC's sound card. This input can be identified by a logo of an arrow going toward the center of what looks like sound waves. Your tape deck is now connected to your PC, and when you use it, the sound will emit from your computer's speakers.