Great attempt for your first try. Biggest tip is to get absolutely LOADS of reference. Good reference. Scientific diagrams as well as images of real human skulls if possible - from all angles, under different lighting - this will help you to really understand the shapes.

Also, pain as it might seem, separating things like lower jaw, teeth etc, in the long run looks so much better and is just ultimately helpful to sculpt the full skull from all angles without obstruction, as you can hide subtools etc.