What's a good linux OS for beginner use, GUI, and file server/web server capabilities? Is Ubuntu good?

Yes, it would be. You don't even have to install it. You can run it live to try it out without installing it. I had previously tried 2 other Linux Distros, but settled on Ubuntu. They have a good support forum to help you with just about anything you might encounter with it's use. Check out the link below.