Keep the fastball, drop the fork, add the two-seam, add any off-speed pitch, and add any breaking pitch you like. Throwing a curveball is harder than it looks. When you throw this pitch, snap your wrist, and hook your arm so that it comes across your body. When you throw a two-seam, try to apply pressure to your pointer finger, and throw it so that the ball stays on your arm side. I don’t know to much about the slider, but I hope my advice helped you.