If you want to become an Athena developer, or just to learn a bit about more about Athena, this page should get you started.
Firstly, ATLAS holds Analysis Software Tutorials several times per year. These contain a lot of information about how to get started as an ATLAS software user, as well as give a brief overview of how Athena works. So this is a good place to get started (especially if you are a member of ATLAS and need to use ATLAS computing resources).
You should also read through the git development tutorial in order to understand in more detail how Athena developers use git (this will be very important once you have code you want to get accepted into Athena)
The next topic, Athena Nightly Builds explains how to use the daily builds of Athena - we also have releases of course (as explained earlier)), but as a developer you will normally be using a nightly in order to have the latest (greatest) version of Athena.
Finally we occasionally have ATLAS Developer Tutorials (for the moment, this page may be restricted to ATLAS members).
In case you need an environment in which to do Athena development, and you do not have access to appropriate local resources, creating your own personal VM in CERN’s Cloud Infrastructure may be a better alternative than lxplus.