Like Squink said, it’s remarkably easy.
You can go nuts and spend hundreds of dollars on high-end audio equipment, but the truth is, most cassette tapes just aren’t up to high-end “high fidelity,” so truthfully, even a pretty inexpensive tape player will work for most cases. I’ve used incredibly cheap Walkman-like devices and gotten pretty acceptable results.
Assuming you’ll do likewise, you’ll need a male mini stereo to male mini stereo cable (Radio Shack still carries these). This has a 1/8-inch “headphone” type plug at both ends. Plug one end into the headphone jack of the Walkman and one end into the line in audio jack on your computer. This usually is indicated with a symbol like this: (( )) with an arrow pointing in toward the center. It has a standardized color code, but I can’t remember what it is and don’t feel like crawling under the desk to look.
Oh, all right.
Light blue. You happy?
Now you’ll need software. You may already have something to use. Most CD burners come with Roxio or Nero and those packages usually have something that will work to record line-in audio. If you don’t, I second Squink suggestion of Audacity, which is a very powerful tool. If you’re looking for simple, try HardDisk Ogg. For either, if memory serves, you’ll need to locate a copy of the lame.dll if you want to make MP3s. This is not hard to find online, but I had mine because I had a copy of Nero Burning ROM.
If you’re using a real tape deck rather than a Walkman, you’ll need Radio Shack’s male mini stereo to male stereo phono cable. Just remember that you do not under an circmstances want to send an amplified signal to that line in jack on your PC, so do not ever connect anything that wants to be connected to loudspeakers there.