About Themes - WordPress
Themes allow you to change the design of your blog at anytime. Changing your theme will not affect the content of your site. Here is a great a place to look at themes: http://wordpress.org/extend/themes/
How to change your theme
Changing the theme is a simple as clicking the "Appearance" tab in the lower left column of the dashboard. Then select a theme by clicking "Activate", listed below each option. If there is any uncertainty about the theme click preview and quickly see how the blog could potentially look.