Hi Pauline
To the best of my knowledge, you need a programme like Audacity, which you can download for free, or CoolEdit which costs money. You then need a cable from your cassette player to your computer. Record it into whichever programme you have chosen, save as whatever kind of fille you want (MP3 takes up less space with only a slight drop in quality and there you go.
I'm in a similar situation to yourself but what puts me off is the time involved as the recordings have to be done in real time – if you have 100 hours of cassettes, it's going to take you 100 hours to record them into your computer.

Good luck

Eddie