Visit Ubuntu.com and click the "Download Ubuntu" link. From there select the version of Ubuntu you would like (32 bit or 64 bit). If you do not know which one to choose, select the 32 bit version to be safe. Further down on the page is the second step which allows you to select your operating system and whether you would like to install from a USB drive or a CD. I would recommend a CD at this point as it is easier and works with more systems.