As mentioned by others, Joomla might be a good option for you, although Wordpress may also work (and it's easier). However, I highly recommend that you check out OpenSourceCMS.com, which hosts demos for many of the free and open source CMS's, blogs, forums, shopping carts, etc. For most of them, you can try out both the admin and the frontend. Spend some time looking them over and then when you find some that you like, investigate them further by going to their websites.
