You have enormous control over the look and feel of your site. It's best to familiarize yourself with some basic concepts.
To choose a theme go to the Control Panel and click on Appearance. From here you can set a theme and choose theme flavors from a number of choices.
Themes provide consistent visual presentation for your site. You may choose from one of many themes included with the Project, or have a custom theme developed for you by an outside designer.
Customizing Your Layout
To customize the webpage layout go to Control Panel, Build, and click on Layout.
Beyond the theme itself you can make choices on where pieces of information should appear throughout your site. You'll be able to drag and drop "widgets" of content to particular pages as you see fit.
Both themes and layout choices are endlessly changeable. You should have fun with your site knowing no content will be lost when you change the look and feel of your site.
For detailed information go to Managing the Look of Your Site.