There are a lot of ways to contribute to Xfce. This section in bugzilla is more dedicated to start writing code for Xfce, help fixing bugs etc. but that is by far the only way to help the Xfce project!

Most of the Xfce development is happening in trunk, that's where the unstable (next) release is. The stable release is maintained in branches. In a branch only bugs are fixed, translations are updated and no translatable strings are changed.

There is a page on the Xfce website with information about getting the source from svn, you can find it here.

Most of the Xfce libraries have API documentation here.

In the Xfce Bugzilla there are a lot of feature requests and bugs. From the development point of view it is very helpful if people help with fixing a bug and/or contribute a patch to fix the issue. This is also a good way to start getting familiar with the code. If you don't know where to start you can always ask that in the bug report, the developer will point to into the right direction (warning; something this will that some time since we're human too). You can also ask this on the xfce-dev mailinglist.

Some people think panel plugins are a good way to start with Xfce too. You can find more about that here.