Here, I'll help you out a bit. Try looking at something that has shading, find your way around gimp, I would suggest experimenting quite a bit with a few different techniques and tools. Then try and put that shading on that thing your looking at onto the comp. It seems hard but it will work out if you're determined enough.
As for the head, I'm sensing you're a beginner, not bad for a beginner. It does need shading, but the outcome that didn't have shading came out ok. You need to touch it up a bit, like fixing those white spots. Once you do that, try and shade. Trying to shade isn't hard, the spots on the metal that are facing down would be darker than the ones facing up. It's basically like that.
Atleast there isn't a seam on the back, we can all agree on that.
Rate would be 6/10 for a beginner. Shading 0/10 obviously. Design would be 7/10. I like the design of the head. Overall with all that together the head is around a 5.5/10
Keep working at it and you will find yourself pretty skilled one day