Before starting contribution it may make sense to get in contact with some xfce experts:

• via IRC: Join #xfce and #xfce4-dev on Freenode
• via some Xfce mailing list - first check if the list is still active !

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:

• Translation – Translate Xfce applications into different languages

• Documentation – add or translate application manuals on this website

• Bugs – report or fix Xfce bugs, provide new features

• Bountysource - Put some money on bugs you want to have fixed
• - as well possible to put a general donation

Once you did some contributions, you may want to grab extra permissions to e.g. release your modifications:

• becoming a xfce-developer - contributor account, git access, releases

Xfce API documentation can be found in the Xfce development Wiki