the first thing for game dev to do

the first thing for game dev to do

Hello, greetings to all the game dev here. As per mention in the title above. I really need to know the basics to start game dev or rather to be called game dev itself. What i should have n do as a starter? First of all, i want to highlight. I want to make a 2d game with 3d background something like puzzle and rpg game at once. 2nd, i like it to be on cross mobile platfrom. Would unity 3D game engine do this for me? 3rd, what are the coding i should be looking for if im looking to implemented geocaching in game, to make it more like MOBLES catching monster method.

Loved,

Zai <3