Good foundation, I would work on line quality and anatomy.
Line quality: don't use such short, sketchy lines. Try moving your arm around with your shoulder. Notice how the movement is a lot more smooth and wide than wrist movement? If you lock your wrist and elbow and move with your shoulder as you draw, you'll get more control over your lines. Practice that by doing a bunch of straight lines and circles.
Anatomy: Proko on YouTube has great free videos. I've had a lot of improvement by watching his videos on a muscle group and then using Line of Action to practice it. Make sure to think of the forms in 3d, and observe your own body.
If you start off with a solid foundation, the 3d should come pretty naturally. Obviously shading will help, but it's more like icing. The big thing is probably overlapping forms. Don't just draw solid contours, make the lines overlap a little. Study other artists to figure out how they do it.
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