Godot is a multiplatform game engine, similar to unity but open source. It runs on Linux/win/osx and exports to win32, win uwp, iOS, Android and Linux.

The best way to learn godot is to follow along a development tutorial on YouTube.

I HIGHLY recommmend Angega's space shooter tutorial:

https://m.youtube.com/playlist?list=PLv3l-oZCXaqkUEqrLsKJIAh...