You have to setup new partitions for Linux on your HDD. Once you do this and install Linux on the new partion (you should make 3 partitions in Linux: root, swap, and \home). So if you have your entire HDD partitioned, you will need to shrink your NTFS partition to allow space for the Linux install. You need to determine how much space you will need for Linux when shrinking the NTFS partition. Then, when you install Linux use the unallocated space (after shrinking) to install Linux. When Linux installs, it will use the GRUB Bootloader and you can select which OS you would like to launch on power up or restart. Besure that you want this as GRUB can be a pain to get rid of if you are unfamiliar with Linux.
Edit: +1 to junkeymonkey...thats the best option in my opinion also. My explanation is if you only have one HDD