Firstly, after you create your new server, a randomly generated password will be provided to you via the UI.  Copy the randomly generated password somewhere safe for future reference.

If you are attempting to login to your server from an external network (i.e. a network outside of your Cloud Servers account), you will need to first add a public IP address to your server.  You then need to permit SSH or RDP access to your server by adding the 'Remote Access' network access group to your server.

To login to a Linux Cloud Server server, use your favourite SSH client (use your built in ssh client on Mac or Linux, or use something like PuTTY or Vandyke CRT on Windows).  Configure your SSH client use the public IP address of your server (or private IP address if accessing from another of your account's Cloud Servers).  The username for your new Linux Cloud Server will be 'root', the password will be the randomly generated password that was generated by the system when you're new server was created.

To login to a Windows Cloud Server, use your favourite Remote Desktop (RDP) client.  Windows users can use their in built RDP client, and Mac / Linux users can obtain their favourite RDP client and use that.  Configure your RDP client to use the public IP address assigned to your server (or private IP address if accessing from another of your account's Cloud Servers).  The username for your new Windows Cloud Server will be 'Administrator', the password will be the randomly generated password that was generated by the system when you're new server was created.