Well, the first thing you need to know is that there are many types of Linux, called "distros". The vast majority of distros are free. A good distro for a beginner to Linux to use is called Ubuntu. (http://www.ubuntu.com) If you've already got Windows installed on your computer, you may consider using Wubi, which installs Ubuntu from inside Windows as a Windows application. (http://www.ubuntu.com/download/desktop/windows-installer) If you don't have Windows on your computer already, you can just follow the instructions to install Ubuntu normally.