That part is easy!
Simply download the wattos file in the downloads section. (When in doubt get the newest version). Then burn it to CD, put it in your computer and reboot. If your computer is setup to detect bootable CD's, it will automatically start and you can try wattOS right from the CD without changing anything on your computer. When you are happy with what you see, there is an "Install" icon right on the desktop. Click on that and follow the prompts and you will quickly have a shiny new operating system!
For more details and help - head to the forums to find others who can help you get things working.