[Comevius]

https://source.android.com/docs/setup

[fgeiger]

I second that, and wholeheartedly recommend the official documentation. It might not be the best entry point, but it is a good reference throughout.

In particular, understanding the repo tool, manifests, etc. is also quite necessary to get started.

Obviously, there is a lot more to working on custom ROMs than that. I've seen a few resources mentioned in other threads that seem great for that.