One tip for learning how to draw is to choose the correct pencil hardness. Another tip is to use natural lighting versus flash photography for references.
If drawings do not have any dark shadows it is because the pencil being used is too hard. Switching from a number 2 pencil to a B, 2B or 4B pencil can help achieve darker lines and shading.
Flash photography has the tendency to flatten features and makes it hard to see an object or model's dimensions. If the person turns slightly to the side and there is sufficient natural lighting, more shapes and skin tone are revealed. It also helps to have the model wear a natural facial expression.
Sketching guidelines of facial features first helps to ensure that they are aligned properly. Unaligned facial features are a common mistake for beginners.
It is also important to choose the right paper, as some types of paper have a surface sheen that is too smooth for the pencil. Thick notepads often have too much give for the right amount of pressure from the pencil. Basic office paper and sketch pads are effective options.
Instead of using circular scribbles for foliage, try more convex shapes. This helps make trees and leaves look more realistic.