Assuming you're on apache, you can create a file called .htaccess at the root of your site, and add these lines
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* /index.php?url=$0 [L,QSA]
This will pass all page requests to index.php. In index.php, you'd want to to parse $_GET['url'] and load up the proper page with include. You'll have to sanitize the input and make sure people aren't including anything they shouldn't. You can get the pieces with something like:
list($controller, $username) = explode('/', $_GET['url']);
The typical MVC structure would use controller/action/id. If "action" is omitted though, as in your example, I'd make it default to "view". As in "view" user's profile. The ID would be the username. Typically, each controller is a class, and each action is a function in that class, any parameters after that are passed into the function. There's also an associated view file with each action.
It's a lot of code to give you a full example (I just coded up an MVC framework!) but that should give you the basics to get started.
Definately check out some other frameworks like CakePHP, Kohana/CodeIgniter if you need more details and code examples.