I've done this before to turn cassette recorded stuff and put them on the computer and visa versa. You'll need a audio cable with plugs on both ends. YOu can go from the headphone jack of the stereo and feed it thru the mic input to the computer. And if the mic input isn't in stereo, get an adapter from Radio Shack to go from 3 conductors to 2. It'll combine the two channels into a mono signal but you'll have it.
I've gone the other way with this. I've taken one of those cd/cassette adapters. The ones that look like a cassette with a wire coming out of it. I've plugged it into the output from the computer to transfer mp3's onto cassette, using a dual tape deck to "dub". It was handy when I used to stock shelves overnites and I could crank my headphones to some really good quality dubs. A lot better than a tape-to-tape dub where you always loose some of the signal quality.
Just use the line-input of your sound card and hook it up to the line-out of your cassette player. Grab the sound card with a decent program.
You can do the same thing with just about any old sound souce including vinyl records. if you decide to encode your old records, do NOT hook up the turntable directly to the sound card input. Because of the RIAA equalization present on records, you'll need to use a turntable pre-amp that can flatten out the RIAA eq into a decent analog audio signal. Then it's striaght into the sound card for capture as MP3, etc.
As a plus, just about any decent software will include some basic functionality for noise reduction in the areas of hiss and scratch/pop.