To get started with Android/Maru OS, you'll need to get familiar with Repo and Version Control with Git.
To initialize your local repository using the Maru OS trees, use a command like this:
repo init -u https://github.com/maruos/manifest.git -b maru-0.7
Then to sync up:
repo sync
See the main Maru OS repository for more info.