You don't need an amp. You can connect the cassette deck straight to the line-in on the computer.

Windows has a built-in recorder which can do the job at a pinch.

When I did this, I used Creative Wave Studio which came with my soundcard. This let me record the cassette, split it into tracks and save them separately. As some of them were recordings of LPs, I also used it to edit out some of the snap, crackle and pop so popular with the vinyl user community