you have a few options.
1. reinstall it and choosing 'no' to the sample data, and amend the basic site, including content, look and feel with html and css.
2. find a template (There are tons of free templates around which you can then modify to your tastes, again with html and css)
3. create a template yourself from scratch - lots of tutorials around for the basics. generally you use html and css for the look and feel, and then include joomla tags to insert the various modules where you want them, like the sidebars, breadcrumb etc etc.
it's a really cool script, but can take a little time to make the site not look like a joomla site =)