If you want a professional looking website, then you may want to consider hiring web developers. There are a lot of companies out there offering complete packages. If you're on a budget, you may also consdier hiring freelance web developers to do the job.
If you want a really simple website, then you can try doing it on your own. Sites like Wordpress or Blogspot offer templates for free. However, you may want to rethink this - if your business is for interior design service, then you might as well make sure that your site looks really good. First impressions last!