Beyond All Reason (BAR), consists of 2 primary components, the lobby (Chobby - https://github.com/beyond-all-reason/BYAR-Chobby) and the game code itself (this repository).
In order to develop the game (this repository) you first need a working install of the lobby/launcher. There are 2 ways to do this:
Follow the guide in the Chobby README. First download a release of Chobby and then launch Chobby, this will automatically download and install the engine and other dependencies.
Download the full BAR application from the website and run it. This is probably what you will have done if you have previously installed and played the game.
Once you have a working install of BAR you need a local development copy of the game code to work with. This code will live in the BAR install directory.
To find the BAR install directory simply open the launcher (not full game) and click the "Open install directory" button. This is one of the 3 buttons (Toggle log and Upload log are the other 2). For Windows installs this might be your user's AppData/Local/Programs/Beyond-All-Reason/data directory.