The method usually depends on the gear being used. So if you could list what equipment and programs you will be using the I can add to the info below. The more detail/info the better ;)

So if you have decent cassete cd playback deck and a good multi I/O sound card you should be able to use a pair of balanced XLR cables to join the two.

If you equipment is a bit lacking in I/O then things start to get more difficult.
If you are using a standard stereo tape/cd player then it will most likely have RCA outputs.
If you are using an internal soundcard (which will sound like crap if used to record!!!) then it will have a 3.5mm jack
So you will need a dual RCA male to stereo 3.5mm male jack cable.

The next step, recording the signal onto the computer and encoding it to MP3. This step is usually different with most audio apps. If you refer to the manual of your chosen app it should tell most of what you need to now about recording audio from an external device.

Hope it helps ;)