You can sign up for an account right here: https://app.backupbird.com/login/register
When you are first logged in, you can now add a server that you wish to backup. You do that by clicking the "Add Server" button, that leads you to a guide. Enter you server details in this guide and press the "add" button in step 3.
You now need to install the backup agent. During this installation, you need your unique server key. Press "Show key" to receive the server key.