Edit: Please read the stuff below, however I have found a nice picture guide for you, which will guide you through the process here: Picture Guide
The other 2 answers are great, always make sure you backup your important stuff before doing anything with your partitions. Even if this means just uploading to say Dropbox, Ubuntu One, or even e-mailing them to yourself.
Remember: Any work that gets done to partions, could potentially cause problems - especially where resizing and creating new partitions are involved.
That being said, usually things go quite smoothly, and its not very often that things go wrong - but you'll be glad you had backups in case things do!
If your new to installing Linux/Ubuntu... then just follow the install disc, and make sure you don't rush things. What you want to keep an eye out for, is trying to get them to dual-boot... you don't want Ubuntu installed onto the full system. It should just detect Windows, and give you the option of how much space you can give to both Windows and Ubuntu, with a slider that you can move about to change them. You should see something like this picture:
If I recall, that's an oldish picture (10.04/10.10). However the new one still has something very similar. So keep an eye out for it, and make sure you pick according to what you want to do (in this case, installing "side-by-side"). Make sure you move the slider along to how much you want to give to both... I wouldn't recommend the "advanced" option, unless you know what your doing (you need some partitioning knowledge for that, along with how linux file things - but these will come with time, the more you use ubuntu).
Hopefully that helps you out.
P.S. Just did a bit of digging and found this screenshot which shows you what you'll see on the 11.04 natty narwhal install disc:
The next screen should give you the sliders I mentioned earlier, or at least something to specify the sizes for each.