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.
Branches
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.