So you want to help develop future versions of the AGS engine and / or editor? The first thing you should do is read the readme, which explains how to get started. There you can find links to information on various development projects, including versions of the AGS engine for many alternate platforms.
Development of AGS happens on our GitHub page. The Git repository not only contains the source code of the AGS editor and engine for Windows, but also ports of the AGS engine to Linux, Android, iOS and PSP (which allow you to play AGS games on these platforms). Instructions for compiling the ports can be found on the GitHub pages.
On the AGS Build Server forum thread you can find nightly builds of the latest version of AGS in development. These are likely to be highly unstable, and should not be used yet on game projects.