If you use 2d (as i am using and recommend as 3d is much more complicated) then i think you should check brackeys' (only for unity engine) youtube channel, which has everything for making games. I definitely recommend it. For art you may use GIMP( i use it) as an photoshop alternative and for pixel art you can use Piskel here as mentioned in an above comment. Btw brackeys also has a video on top down shooting and movement.