Seconding what the Shook said. Everyone is an amateur at first, only with time do you see yourself improving. With art especially you'll se a painfully slow progress, even with focused studies. The trick is sticking with it long enough to see yourself improve and not suck. I've made great progress since I first started digital but I still feel incredibly inadequate and unskilled. Yet I continue to doodle and dabble and paint, not only because I want to improve but because it's damn fun to do.
And that's the most important thing about it I guess, it doesn't matter how much it sucks or how slowly you improve as long as you're having fun with it
Anyways, about the apple Shook. Since you're watching CTRLPaint already I suggest you check out the blending videos. They should make it look more natural and the tranistions should give away that it's a round object. Also the opacity is best left as a hard setting (in that it doesn't change with pressure), atleast until you get used to the tablet itself. Having to fiddle with the right pressure while also trying to nail the line isn't exactly the easiest of things at first.
Also about the brush size, blocking in colours is best done with as large a brush as possible since you don't want details or messyness, just a rough blotch of colour where you want it. Then you size down as you gradually work on details and other stuff.