Here's an edit I made (Note: I'm no pro, not even close). But you need to look more to your reference, and if you don't have one, google it. Here's is the one I used:
link.
And here is the incomplete edit.

- The shape of the one you made looks very weird.
- The skull you made (something I didn't edit) has three central incisors.
- The skull is looking to the right, but we can see the skull's left cheek/jaw looking frontwards.
- The eye sockets are too triangular.
- The highlight is very strong and seems unnatural. (Mine isn't perfect, I know, is pretty awful

, but maybe you'll get the idea)
And finally as twodayslate said, finish the skull, then start adding details/accessories.