First, install it! :) Or, if building from source is more your cup of tea, jump to Section 4.1: building from source.
To get started in an existing project using mdm, your first steps are covered in Section 2.1: Syncing Dependencies.
To start a new project using mdm, or start contributing changes to dependencies in an existing project, you'll want to know about Section 2.2: Adding, Changing, and Removing Dependencies.
If you're ready to start publishing releases using mdm, Chapter 3: Using to Release covers setting up releases for a new project, performing a release, and how to host and share releases.
If you prefer to learn about a system by studying the data structures and theory up front, Chapter 4: Advanced Topics and Theorycraft is just for you.
Happy hacking :)