Where do I start if I want to write a wayland compositor?

I can only recommend to take a look at the KWayland sources (especially if you're interested in how the wayland protocol maps to an object oriented world).
KWayland is an object oriented (C++/Qt) wrapper around the wayland client and server libraries and it also includes a minimal server (tests/renderingservertest.cpp).

I wrote a minimal wayland compositor that runs on top of X11 with the hope that it might be useful for other people who want to learn how to write a wayland compositor.
It's about 500 lines of code that are really just the minimal code that is needed to host multiple instances of weston-terminal and move them around.

Take a look at the Small Wayland Compositor. You can either use it as library to build upon, or check out how it does things internally if you want to start from scratch.