GParted is software that can resize or create hard drive partitions without losing any of your data. You can either get their LiveCD and boot to that just to do partition work, or if you are installing Ubuntu, just use the GParted that is already part of that installation process. If you do it the Ubuntu way, you don't even have to know or care that it is called GParted. It's just part of the installation process where they ask you how you want to partition your drives.