Ubuntu is the most popular and thus the most supported. If you wait until april the newest version will be out. You can try it now though if you want, upgrading is easy. You can download the iso from ubuntu.com. The disc has a paritioning tool that is kinda confusing if you arent used to it, so if you need any help feel free to ask. The cool thing is that with the install cd you can basically run the entire operating system without installing it. Its called a Live Cd. So if you run into a problem or a question while you are installing you can just open up firefox and ask someone.
Some people say that Kubuntu (its the same operating system, just a different window manager. If you dont know what i mean dont worry about it) is more windows-like, but i havent noticed. Kubuntu has alot more eye-candy and almost everything is easily customizable. Neither Ubuntu or Kubuntu is better, its just preference. They both run the same* programs and are really the same OS.
*There are certain programs that work better with Kubuntu than Ubuntu, but they all come pre-installed anyway. Dont worry about it
