There are similarities in certain functionality (like: both tools enable creation of 'sites', have 'personalization', use 'roles', 'templates', etc.), but from the developers' point of view they are quite different - WebCenter Portal is based on ADF (Application Develoment Framework) whilst WebCenter Sites has its own libraries. The least common denominator, therefore, is "Java, JavaServer Pages (JSP), XML, and HTML".
If you know ADF, WebCenter Framework (the foundation for WebCenter Portal Applications or WebCenter Portal: Spaces) should be a relatively easy step for you.
On the other hand, it does not mean that Sites (if that's what you should do) is a rocket-science. You just need to learn the logic of Sites. I'm afraid I'm not much help there - try to get some references how to start with Sites in their forum.