Welcome to Linux Questions!
The operating system known as Linux or GNU Linux is actually a "Distribution" of Linux Kernel, libraries, File managers, applications, etc. There are Various distributions just to confuse you; Ubuntu, Mint, Slackware, Mandriva, Centos, Fedora, Puppy, etc, just to mention a few. You normally download an ISO image file which you use to "burn from image" with your favourite CD burning software to produce a bootable CD.
A lot of these CDs are now known as "Live" CDs as you can boot and run Linux in memory without damaging your existing operating system install. Usefull when looking for a distribution which suits your needs, if you like it, you can hit the install button to be guided through the install process.
Ubuntu seems to be one of the most popular distributions at present, you can download it from here:
If you want to install, back up anything you want to keep on your laptop just in case!