If there is free space on the disk, then simply let the Windows installer do the partitioning. Be aware that if you install Windows after something else, you may need to do some tricks with the bootloader (but it's all doable.)
Depending on how big your drive is, you **might** want to consider a second drive.
Tell us more about your configuration--size of drive, existing partitions, etc. (Post the output of "fdisk -l" (run as root).
I won't mention backup important data since I'm sure you already know that....