If you don't have any partitions at present then defrag Windows, this isn't truly necessary but it may make things somewhat safer
Download the Gparted livecd,from here
You'll need to unmount the windows drive if you wish to shrink it, but it's all straightforward and well explained on Gparted's site see here
This'll give you an easy and safe method for creating a partition for Linux to install on.
Pop Linux Mint in your drive and off you go
Check out the wiki
if you want to read up on anything before continuing
As Slocker said, if you can back up then do so, it never hurts.
I've never had any issues resizing NTFS but thats no guarantee by any stretch.