That's actually a great question. While using the application may be very easy, it's not often clear, when faced with a blank slate, how to start planning a project.
So we made this short instructional video. It's three-minutes in length, but it tries to cover a lot. Not only how to get started, but also best practices and techniques we've been learning by using Gigantt ourselves.
The video is doubly awesome because it's self-referential. You'll have to see what that means yourselves...
Don't forget our ever-growing help wiki.