ToastedToad outlines the general mounting procedure but here's a more detailed account for newbies:
(1) Open a terminal window, "su" to root, and do "fdisk -l" to find out how Debian sees your vfat partition.
(2) Open /etc/fstab with the nano text editor ("nano /etc/fstab") and add a line for the vfat partition, something like this:
/dev/hda1 /mnt/win vfat user,noauto,quiet,umask=0 0 0
(You can use the above line if "fdisk -l" shows that your vfat partition is /dev/hda1.) Exit nano saving changes (Ctrl-x, y, Enter).
(3) Create a mount point for the vfat partition ("mkdir /mnt/win").
(4) Become a normal user by hitting Ctrl-d. Mount the vfat partition with "mount /mnt/win". Now you should be able to use your vfat partition from Debian, and you can unmount it as normal user with "umount /mnt/win". (Dropping the "noauto" switch from fstab should make the vfat partition to be automatically mounted at boot time.)