General instructions on using git can be found on fd.o's Git pages.
For a quick getting started guide, there is the Everyday Git With 20 Commands Or So guide from the Git homepage. For more in depth git documentation, see the resources on the Git community documentation page.
A quick guide on how to get the complete xorg modular tree.
Instead of using a kernel-style personal repository model for long-lived branches, we favour actually using branches (e.g. mpx, input-hotplug, pci-rework, randr-1.2) within the repository. Ideally, these should not be created on a whim, as they will stick around forever once pushed.