You can see a sample hosts.allow file at this URL:
http://www.patpro.net/images/hosts.allow
Copy this file into /etc/ and edit it. How to configure the file is pretty obvious as soon as you take a look into it so I don't detail this part (the file is commented with basic instructions).
WARNING: Be really careful when working on a remote machine, messing up the hosts.allow can prevent you from connecting again to that computer.

