There's ubuntu installers in the market. However it's best if you want to try it out to use a pc. Just go to
Home | Ubuntu and download the desktop 32 (or 64 bit if your machine is 64bit). The 32 bit one works guaranteed though. Just download the .ISO cd image and burn it to a disk, you can boot your pc by leaving the cdrom inside (sometimes you need to press a key F2/F12/ESC) you can run it right off the disc without installing it. It's pretty dope for when your pc's not working and you need a working computer to copy stuff off your dead windows install. It runs much better than windows on some pc's. Just don't expect to run netflix or itunes. Everything else pretty much works and comes on the cd though, like an office suite, image and movie players etc... unlike windows.