You need CrossDos on your Amiga and a 720k PC disk. Otherwise you may have to install a CF Flash memory reader in your systems but that is too complicated for now.
First do you have Amiga Workbench version 2 or better?
If so you must have CrossDos in Devs:dosdrivers/PC0 . Just drag the icon from sys:storage/dosdrivers/PC0 to the other directory and restart the Amiga.
When you put a medium density disk in the Amiga, there should be two icons that appear: DF0: and PC0: .
Click on PC0: and full format it there (not quick format). Use LHA to compress any disk images you need to transfer since PC disks don't hold as much uncompressed data as an Amiga disk.
Copy it to the PC0: disk and use
http://aminet.net/util/arc/lhant.lha to unpack the lha files on the PC. Note: You'll have to decompress that file on the Amiga and transfer it over to the PC first. It will run from the command prompt under Windows XP.
If you need more help, just post back here.