In order to style your WordPress site or blog, you need to install a theme.
By default, every WordPress 3.X.X installation comes with a theme called Twenty Ten. It offers a modifiable header image and background, along with a single widgetized sizebar on the right.
If you want a different look and feel, more or fewer sidebars, more control over the header, an alternate layout, or a different navigation menu, you might want to browse the WordPress theme options. You can access themes under “Appearance” in the Admin Panel. Once in Themes, click on the top tab “Install Themes”, and you’ll be taken to a search window. You can search or browse the themes in the WordPress directory here.
Once you find one you like, you can preview it first, bringing up a larger view of the theme. Then click “install”.
If you find a theme on the web that is not in the directory (particularly premium themes), download the theme from its website. Then go to the Themes directory in your Admin Panel and click “upload” and find the zipped file on your hard drive. That will install the theme and make it available to you.
Once you’ve chosen a theme to try and have installed it, return to the “Manage Themes” tab and activate the one you want to use. Then go view your actual, live site (I recommend doing so in another window/tab) to see if you like the new look.
Note: you can install/uninstall as many themes as you like without hurting anything, so don’t be afraid to try out different themes until you find one that works.
Helpful tip: Woo Themes has a video on their site that explains how to install a theme.