Adding to the nice answers of my colleagues, where you see
- the importance of the Widgets
- the extensive documentation in the Codex
- the use of free Plugins to make tasks easier and add functionality to your site
- using this Stack as source of in-depth information
I'll add that you have lots of Free Themes.
And many of them will provide an Options Page, where you can customize the look and content of many parts of the site (header and footer, for example).
There are Market Places where you'll find themes completely packed with features. But that's not necessarily a good thing. Contrary to what happens in WordPress.org, coding standards are not always followed by these themes. And sometimes this can bring severe headaches.
I believe the most famous is ThemeForest, and they recently have been taking steps to improve the quality of what is sold there.
Then you have the Theme Houses, like Woo, Elegant Themes, Graph Paper Press, to name a few, where the code is so well done as the aesthetics.
Then, Theme Frameworks. For which this article provides an overview.
Whatever you choose, stay away from shady Theme providers.