Do you createing mods for games?
I want to learn how to create cars for GTA 5 just like gtaall.com/gta-5/cars that.
Where should I start as a beginner?
Develop games in your browser. Powerful, performant & highly capable.
Google would be my first choice
I think that the Scirra forum is the wrong place - here it's about small 2D games. GTA5 is pretty much the opposite.