There are several methods of obtaining Ubuntu.
Downloading the Ubuntu ISO
Go to the Official Download Page
Select the Ubuntu Desktop version that you wish to install:
If you have a recent computer (>2010), or if you know that your computer is 64bit, you should select the 64bit version for better performance and UEFI compatibility.
Else, you can select the 32bit version.
Click the Get Ubuntu button to begin the download of the ISO file.
Either burn the ISO on to a blank DVD or burn the ISO on to a USB key.