you have to make a texture so you must go to 'canvas/new image and pick the resolution you want 512 by 512 here. name it if you want (i already did this and should have mentioned it in the last post, it is square like this because th resolution is 512 by 512.
anyway, when you have hat sorted paint something on it. in this case i painted some muddy looking grass and a flower.
take it easy and life will be easy