I have several ideas, but dont know the equipment you do have, so here goes. If you manage to blow up your stereo or computer, please dont blame me - you do this at your own risk. I have personally used #1 to convert 'old' music tapes to CD.
1. If you have a simple tape recorder/ walkman, etc (or a headphone jack on the stereo). Radio shack sells stereo patch cords, you would most likely want the 1/8" male stereo plug to 1/8" male stereo plug type. Connect it between the device and the audio input on your computer. See WARNING below about volume control.
2. If you have a home theatre, detachable speakers on a stereo or such, use the same type of cable, but this time you will only want the 1/8" stereo jack on the computer end. The other end should be bare wires. The cables typically have a right wire, a left wire and a shielding jacket around both. The individual wires would get connected to the + side of the speaker (or red connection). The shielding 'wire' would get connected to the - or black connection. You will need a small jumper to connect both negative terminals. If the tape is mono vice stereo, just pick a side. WARNING - Your stereo can probably put out more power than your computer can take. start with the volume LOW (as in almost off) and gradually raise it until you get a signal or sound out of your computer. Popping, static, sparks and smoke are all bad thing and you should pull the plug if any of these things occur. Likewise, double check your connections so you dont short out your stereo and blow that up.
2a. Use the RCA jacks on the back of the stereo. Again Radioshack has a patch cord that goes from RCA to 1/8" stereo, it looks like a "Y". (Same WARNING above applies)
3. Play the tape in a quiet room into a microphone attached to your computer (this would result in low quality most likely)