The code is hosted on https://github.com/MariaDB/server. You can branch the latest code from there, and you can also push your own changes as a new branch that can be shared with others.
Building the code from source is done with standard Unix tools: CMake (or autotools for MariaDB versions below 5.5), Gnu Make, GCC (or other C/C++ compiler on some systems). On Windows, CMake and Visual Studio are used.
The current state of the source with respect to build/test failures can be seen in buildbot.
For project management and bug tracking, we use JIRA.
The source page has links to instructions on setting up a full development environment, if you are interested.