how do i transfer cassette tape using garage band

First connect the cassette player to the Mac. How you do this depends on what output your cassette player has (1/3" mini plug or RCA stereo cables), and what input your Mac has (Line-in? USB only?). Get the cable with the correct plugs for both ends.

After connecting the cable, play a cassette. If you can't hear it in the Mac, you will need to make sure GarageBand is set to use the connected audio interface (analog or USB) for input. If you are doing it correctly, playing a cassette should show up in the GarageBand level meter on the track that is armed to record. As with any recording, you might have to adjust the output level of the player or the input level in GarageBand to set a strong signal that isn't clipping.

After recording you can clean up the track in GarageBand, export it and burn it to CD. I actually use other software (not GarageBand), so I don't know the details in GarageBand.

This article might help with some of the details outside GarageBand:

Recording Cassette Tapes into a Computer