Find a cassette player. You might have a deck in the attic, or you could borrow one from a friend. Even a Walkman will get the job done.
Figure out what kinds of connections the cassette player's line-out (headphone) jack and your computer's or soundcard's line-in jack require. Most computers require 3.5 mm connections, but your cassette player might need something larger.
Procure a male-to-male audio cable that fits your needs. If you have to buy one, they're only about $5. You can also purchase adapters if you have a male-to-male cable that isn't the right size.
Download Audacity. Audacity is a free, open-source audio editor that's available for all operating systems. Go to audacity.soundforge.net and click the appropriate "Download Audacity" link for your operating system. You'll find installation instructions when you click the link. Install and open Audacity.
Attach the cable to both your cassette player's line-out jack and your computer's line-in jack.
Open the "Edit" menu in Audacity. Select "Preferences." Under "Devices," select "Microphone."
Select a tape. Put it in the cassette player. In Audacity, press the red record button, then press play on your cassette player.
Press the yellow stop button in Audacity when the cassette stops.
Open Audacity's "File" menu and click "Export to WAV." Your digital copy of your analog tape is now saved to your computer.