To get started with Android/LineageOS, you'll need to get familiar with Source Control Tools.
To initialize your local repository using the LineageOS trees, use a command like this:
repo init --depth=1 --no-repo-verify -u https://github.com/LineageOS-T/android.git -b lineage-20.0 --git-lfs -g default,-mips,-darwin,-notdefault
Then to sync up:
repo sync -c --no-clone-bundle --no-tags --optimized-fetch --prune --force-sync -j8