Make a full backup of your site before unleashing the developer into it (and he has nothing to do with your domain name). If anything goes wrong, revert to the backups pre-developer.
Not everybody can be a developer or take time to learn the ropes, so finding developers are a necessity. But it helps if you learn the ropes as well so you can communicate better with the designer/developer or at least be on an educated lookout if they're up to no good.