Based on your question, it sounds like we need to start at the beginning with how to make believable simple 3d shapes (cubes, spheres, cylinders, pyramids, etc.). And then add shadow for added realism. Even the most complex subject to draw (the human figure) is made up of a combination of simple shapes.
It will take practice. But, you may be surprised how good you can become very quickly with patience. You may find it encouraging to know that even many of the masters are still struggling to get better. Art is not a destination. It is a journey.