You didn't say what version of OWA but regardless you have your work cut out for you. First, you need to have Secure Sockets Layer (SSL) installed on your OWA server. This article will tell you how to do that: http://support.microsoft.com/support/kb/articles/q234/0/22.asp.
Then, see http://support.microsoft.com/support/kb/articles/Q268/4/19.ASP for details on what else you need to do.
This was first published in May 2001