Click around to get familiar with how Webgarden administration works. Once you've explored things such as the Menus, Sections, Documents, Multimedia features, etc., think about what structure you'd like your website to have. One option is to first draw a basic scheme on a paper, and then apply your ideas using Webgarden.
Be sure to delete any prefilled content (like where it says "Website title," for example), and replace it with plenty of content of your own.
Once you have some content filled in, choose a design from one of our many templates. It's best to choose a design based on the colors and fonts you like.
Most importantly, don't be afraid to try new things. You can change it anytime.