In order to back up a database you can simply click the 'Back up Database' option in the control panel.
This takes you to the following page where by you need to enter the relevant database details in order for the backup to be initiated.
Database Type: Generally this will be mySQL
Port: Which port does the database need to connect on
Hostname: What is the external hostname of the database
Database Name: The name of the database
Database Username: The username for the database user to access the data
Database User Password: The associated password to work alongside the username
This page also gives you the following instructions on allowing your database to be accessed by the backup service.