This page has general information on X development, including how to get started.
A common misconception is that developers need to understand all of X to get started. This is not true, and even of the "core" developers, only few actually know all the pieces that put X together. The best way to get started is to simply pick a problem and give it a try.
Documentation
Using git: Information on using git.
Making a release: How to make an individual module release.
Writing documentation: How to document your work in X.
The devPrivates system: All about the private storage system in the server.
Grab processing: How input grabs are dealt with in the server.
Cursor handling: Introduction into DIX cursor handling.
Input event processing: How input events are dealt with in the server.
Security: How to make use of the security facilities within X.
Performance: A fairly detailed document explaining what our performance problems actually are, and things that aren't performance problems at all.
Xorg input driver HOWTO: How to write an input driver for X.Org.
Xorg video driver HOWTO: How to write a basic video driver for X.Org.
Documentation being converted to DocBook: What needs to be converted and what is being converted
Glossary: Various terms used in X, and what they mean.
Dri Wiki: Information about using the Direct Rendering Infrastructure (DRI) with X.
Development/git: How Build Xorg from git versions or the bleeding edge version, (need many improvement).
How video cards work: A conceptual overview.
Tools
Server debugging: How to debug the X server with gdb.
Using ctags to find functions: Some tips to find a function in vim.
Using Eclipse: Some tips to use Eclipse to develop X.Org.
Various Projects
Pointer acceleration A better pointer acceleration for mice and the like
PciReworkProposal: API documentation for libpciaccess and other ideas
PciReworkHowto: How to fix a video driver to use libpciaccess
VgaArbiter: Routing VGA instructions correctly.
Helping out
ContributionHOWTO: How to get started contributing bug reports, patches, fixes, or even entire drivers.
Help us: Things that need doing.
Janitor Xorg Janitor project page