but it can sometimes be a little involved.
Try this first:
FTP into your server space, and take a look for a file called wp-config.php
If you see it, open it in Notepad, and look for some code that will probably go like this:
// ** MySQL settings ** //
define('DB_NAME', 'wordpress'); // The name of the database
define('DB_USER', 'username'); // Your MySQL username
define('DB_PASSWORD', 'password'); // ...and password
With luck you'll see your username and password there.