In addition to understanding how to utilize the Upgrade Assistant in order to migrate some of the core components to 12c, you should also have an understanding of the following issues before you embark on an upgrade of your current implementation:
Understand the original 11g architecture and plan how you will migrate across. This is especially important if the current implementation is in a clustered environment or has a lot of customization.
Understand and plan for the new skillsets required when implementing and supporting 12c. For example, WebLogic is a new middleware component in 12c that was not previously part of the product set.
It may be necessary to create a proof of concept, and/or, at the very least, carry out a dry run. This will give you an opportunity to perform a consistency check for the upgraded RPD. If this highlights new issues in the RPD and Web Catalog, a better understanding of how much time/resources is needed to x these will be reached...