Stay with pen and paper until you grasp foundation to a considerable extent, then move to SAI (don't use the stabilizer), later you can move to photoshop.
I say this because there's nothing that furthers your progress more than learning foundation with pen and paper, moving to SAI afterwards is ideal, since it's just for straight up drawing, so the transition will be a lot easier, finally, leaving photoshop at the end is something that imo is best for your development as an artist, mainly because, even though it's industry standard, is not just for drawing, so adapting to it will be incredibly hard and tiresome, even more if this is your first experience with digital stuff and you still have your fundies on the way.