Hi Kathryn,
If you're thinking about transitioning to WordPress, I recommend installing WordPress to a temporary folder, such as yourdomain.com/wordpress. If you use Fantastico to install WordPress, during the installation set Install in directory to wordpress. This will put WordPress at yourdomain.com/wordpress instead of simply yourdomain.com.
If you put WordPress into a separate folder as described above, it will allow you to build your WordPress site so that it does not conflict with your current website.
After you are finished with your WordPress site and want it to go live, you'll need to uninstall your current software. After uninstalling your current software, you can then setup your site so that WordPress is loaded from your "wordpress" subdirectory.
I understand this can be a bit daunting if you're new to all this. If you get stuck along the way, feel free to ask additional questions, we're more than happy to help.
Thanks,
- Brad