You should provide a bit more information in your question such as what load balancer are you using, is it going straight to WAS or via IHS, how your web application is configured (web.xml).
So here are some general hints that might be useful for you.
If you already have security configured and login form correctly being displayed in http, you just need to add the following to web.xml:
If your load balancer is configured to offload SSL and froward request to WebSphere using plain http, then you need to configure WebSphere to be aware of that. This is done by configuring httpsIndicatorHeader custom property, and adding custom header in load balancer.