Once you have hosting and a domain then you just need to choose a website backend. The far and away most popular is Wordpress. It is free to download, most hosting providers offer a simple one step installer and there are tons of both free and premium themes (how the site looks) and plugins (added functionality). If you search (I am too new to put links in replies still) for buddypress you will find a free wordpress extension that can turn your site into a social site.
There are alternatives like Joomla, Drupal, Concrete5, Umbraco (.NET) and many, many more, but you will find Wordpress to be both the easiest system to learn and the most supported (number of free or very inexpensive plugins/themes). Good Luck!