Creating a New Theme

We’ve been testing WP for a few weeks and love it. Now we want to create a theme so that our blog will have the same look and feel as our other sites. What’s the best way to create a new theme without messing up the live blog? Is there a way to create a beta version of our blog to use for creating the new theme? The thing is we don’t want to break our blog while we develop the new theme. Any ideas? Thanks!