^^Use a cable with 3.5mm jacks on both sides. Plug one side into output of a walkman/casette player and other in AUDIO IN port of your computer. Play the casette in the player and use sound recorder in windows or a software like audacity to record in computer. Later you can convert them to any format you like.
You would have found the solution at
Cassette -> mp3? had you tried to use the search feature of the forum.