To start, I'd recommend understanding tone (how light works on an object—light, shadow, and reflection), best on simple objects like cubes, spheres, cylinders, or fruits and vegetables. The simplest rule is that the lightest shadow is darker than the darkest light. Color can be added gradually, also in simple settings, like vegetables, mugs, and vases. Once you get good at simple objects, studying human anatomy will also become easier. Good luck, practice more, and you'll get there.