Securing your server is something very important to do. Cody Littlewood wrote an article on how do to this is 9 easy steps. Those steps are a good start at securing your Ubuntu server.
My First 5 Minutes on a Server, by Bryan Kennedy, is an excellent intro into securing a server against most attacks. We have a few modifications to his approach that we wanted to document as part of our efforts of externalizing our processes and best practices. We also wanted to spend a bit more time explaining a few things that younger engineers may benefit from.