This probably belongs in the audio discussion area. But anyway -- you play the cassette tape with a cassete deck or walkman... run the output through whatever converter jacks or cords you need to use, into the input or inputs of the sound card on your computer (assuming you have a sound card and not just built-in sound on your computer's motherboard). Using at least Windows Sound Recorder (you'll find it under All Programs if you look hard enough) or better yet, one of the many other sound recording programs out there, record a file on your hard drive while the cassette is playing. Record mode should be 48Khz stereo PCM. The file saved will be in .WAV format. This can then be burned onto a CD-R using Nero, Roxio, etc. If you want it to be playable on audio CD players make sure you burn it as an audio CD, not a data CD. Good luck. Warning, CD quality audio (plus imperfect quality or unclean heads in the cassette deck, bad jacks, inexpensive soundcard etc) will make the results sound WORSE, not better, than the cassette did. It's a fun experiment, though. You can archive old vinyl LPs this way too.
How do i know what converter jacks to use and how do i know if the sound is built into the motherboard. Are there any easier instructions?