This answer is relevant if your website has been designed and built by BT. View FAQs for Web Hosting if you built your website. If you're not sure which website package you have, please read How to get support for your website.
Your website is built on a platform called Joomla. You can update content on your site and create new pages by going to your website and adding /administrator at the end of the URL. Next, when prompted, log in with the details your website designer sent you.