You have enormous control over the look and feel of your site. While more details documentation is available on the following points, it's good to familiarize with some basic concepts.
To choose a theme go to Administrative Toolbar and click on Appearance. There you can set a theme and choose theme flavors from a number of choices. Themes provide consistent visual presentation for your site.
CUSTOMIZING YOUR LAYOUT
To customize the webpage layout go to Administrative Toolbar, 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 add "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.