When installing more than one OS, each needs it's own disk partition.
Actually a Linux install can be split onto numerous disk partitions, but a swap and / (root) are the usual minimum. Linux does not require a primary partition.
Your choices are either a second hard drive for Linux or resizing your current Windows partition.
Remember you need to look carefully for resizing tools for NTFS.
While you are resizing, it is not a bad idea to create a FAT32 partition for easy file transfer. Linux does not write to NTFS well yet (possible file corruption). Both Linux and Windows can read and write to FAT32.
A good idea for testing Linux is to try one of the "Live CDs".
is one of the best, there are lots of others.
Any of the major distributions should be okay, check their hardware compatibility page.
Most also have some of their install documentation online.