The official ISOs carry a bootloader that can load them off a USB device OR a CD. Simply use
Unetbootin to write the ISO. It supports Windows (personally tested) also supports Linux and Mac. Or, if you already have a Ubuntu machine, there's a "startup disk creator" application you can find in the dash.
You can download the iso from
http://www.ubuntu.com/download/desktopLTS 14.04.2 is the default-selected version, and is a very good choice unless you either want really bleeding edge stuff, or have a known technical requirement for a different version. You just need to pick 32-bit or 64-bit (for most modern machines, 64-bit is likely) and click the top Download button (on the purple background). Then, make a donation (or click "no thanks" at the bottom, because they don't take Bitcoin) on the next page, and your download will start.