I regret to say that converting your cassettes into mp3 files or whatever will take as tong as it takes to listen to them normally as they will have to be played back into your computer at x1 speed. Any cassette player with a low level output can be used - plug this into the audio in socket on your computer and then you need some recording software to turn this into the required .wav or .mp3 format. I think you can do this with Audacity which is free.
P.P.