If you have a cassette player (with headphone port) and computer, you already have almost all you need.
1. Purchase a cable with a headphone plug on both ends. Walmart has them. Like, three bucks.
2. Download and install Audacity, a music editor.
3. Use that cable to connect your cassette player headphone port to your computer mic port.
4. Open Audacity and hit record, then hit play on the cassette player.
5. Edit the resulting recording in your computer and export as a computer audio file.