You could start by reading the Linux Partition HOWTO:
http://tldp.org/HOWTO/Partition/. Read it carefully before you begin.
Use windows partitioning tools such as Partition Magic to resize you windows partition to make free space at the end of windows partition. Don't worry about formating. That will be done when you install Linux.
There are excellent partitioning tools in Linux, but I'm not certain that they resize the filesystem of a partition as part ot the process of making free space for another partition. There are also filesystem resizing tools, but again, does one need to use both and in what order to resize a partition with Linux tools? I don't know. So, use windows tools if you have them to make free space, then use Linux tools during installation to make a new partition and filesystem for Linux in the free space.
