To get started with Android/LineageOS, you'll need to get familiar with Git and Repo.

On the LineageOS Wiki, you will find additional useful information. (Just pick any device to find build instructions, but consider the device-specific statements not applicable). To build LineageOS 12.1, you need OpenJDK 1.7, which however is not part of the standard package repositories in a modern Linux distribution. See the Ask Ubuntu question How do I install openjdk 7 on Ubuntu 16.04 or higher?. Skip the "PPA" suggestion even if it is the most upvoted and implement one of the next options.

repo init -u https://github.com/cmtank/android.git -b cm-12.1 --groups=all,-notdefault,-darwin,-x86,-mips
repo sync --no-tags

Note: If you use a MAC to build, omit the -darwin in above repo init statement.

make clean  
repo sync --no-tags 
source build/envsetup.sh  
brunch tank  

Contributions (i.e. pull requests) are always welcome.

• We have an own Thread on XDA