Before starting contribution it may make sense to get in contact with some xfce experts:
It as well might help to check the current roadmap in the Xfce development Wiki in order to get an overview on ongoing development.
There are different ways in which you can contribute to xfce:
Once you did some contributions, you may want to grab extra permissions to e.g. release your modifications:
Xfce API documentation can be found in the Xfce development Wiki