To get started with Android/droid-ng, you'll need to get familiar with Repo and Version Control with Git.
To initialize your local repository using the droid-ng trees, use a command like this:
repo init -u https://github.com/droid-ng/android -b ng-v4 --git-lfs
Then to sync up:
repo sync -c -j$(nproc --all) --force-sync --no-clone-bundle --no-tags