You'll need a USB drive (4GB will do nicely) and a program to create a bootable USB stick from an iso (I recommend Rufus).
Download the Ubuntu .iso here
Once downloaded, run Rufus with your USB drive plugged in. Select the USB drive in the Device menu, click on the little disk image button next to the Create a bootable disk using and select the iso you just downloaded. Then click start. It will ask you if you really want to format the drive, which you need to do, so make sure there's no important info on it. Rufus will take about 7 minutes to complete (on my system anyway).
Boot off of the USB drive
Once the USB loads, click any button, then select the language. You now have several options Ubuntu Live Install Boot Menu
You can either Install right away or Try Ubuntu and then install. Once you decide to install, Ubuntu will give you the instructions you need.