My tips would be to get rid of the white blotches near to the black lines. Those kind of stand out. And also dont be afraid to show deep contrast between shadows and highlights. The contrast helps to create the illusion of realism. Hope this helps.
P.S Biggest tip - Keep at it, don't get discouraged if it seems like you're going no where. You'd be suprised at how good you get at it after a while.
P.S.S Tutorials can be very helpful. Try to find as many as you can from different places. You'll learn valuable things quickly, but be careful not to fall into the trap of 'copying' their style exactly.
P.S.S.S Graphic Tablets work wonders
Good Luck!
