You will need to repartition the ntfs partition so you can make room for linux, or you could buy a seperate hard drive for linux, if you have the money.
I have used partition magic in the past to repartition windows. It works pretty good and is pretty self explanatory.
If you don't have the money to buy partition magic, you could download gparted
, a live distro that has the Gnome Partition Editor application on it. I have never used this, but found it with a google search.
Edit: I don't think I fully answered your question, yet just started.
Depending on what you want to do with this version of linux (learning, another os that you will store files on, etc) will determine how much space you want to create for linux. I would recommend 5-10 gigs at least. This will give you enough room to install the os, and have some room for files. Once you get space cleaned up, you can either have the installer automatically setup your partition sizes within linux, or you can specify them yourself. A partitioning guide for linux can be found here
I think that should get you started down the right path.