Use the FDISK command in a similar manner to how you FORMAT. It is quite user friendly and will allow you to make as many partitions of different (or the same) sizes as you want.
How big a partition? How big is your HDD? Mine is only 3Gb so I have it in 2 halves, one for Windoze and other sorts of Apps, the other for documents and my own stuff. Really it depends on how much you want to give, but you probably need at least 200Mb.
HTH