Open your computer's audio recording program. You can find several programs online, but most computers already have an audio recording program installed. In the "Start" menu, go to "All Programs," highlight "Accessories," scroll over "Entertainment" and click on "Sound Recorder."
Plug one end of your audio cable into the headphone jack on your tape recorder and plug the other end into the audio-in port on your computer. You can get an audio cable at any electronics store. You have to make sure that the cable is compatible with your tape recorder. Bring your recorder to the store, and the staff will find you what you need. Most jacks are either 1/4- or 1/8-inch.
Prepare your recorder for playback. However you do this will depend on your specific tape recorder. You'll probably just have to rewind the tape and make sure it is on the side you want it to be.
Press "Record" on your sound recorder and then press play on your tape player. Your tape is now being recorded onto your computer.