You will need to learn the Grayscale or the Grey Value Scale. These are the ten colors ranging from black to medium grey to light grey to white. Check out "The Color Wheel Company" website and search for the product called the "Gray Scale And Value Finder" chart. This chart will help you learn how to draw the ten colors that will need to be made with your graphite pencils.
In order to draw/paint something realistic you must understand how the subject is lighted:
1. Side Lighting
2. Three Quarter/Conventional Lighting
3. Frontal/Front Lighting
4. Top/Overhead Lighting
5. Back/Rear/Rim Lighting
6. Diffused Lighting
7. Stage Lighting.
As you observe how the light falls on the object, your next step is to draw/paint it. The 7 lighting arrangements mentioned above are shaded in 7 different ways on an object using 5 values. In order to make something look realistic, you must use a variety of values from light to dark.
Learn to draw/paint the 5 values on an object:
1. Shadow
2. Halftone
3. Light
4. Highlight
5. Reflected Light
(The above 5 terms are found in Darren Rousar book)
Lee Hammond book uses these 5 terms:
1. Cast Shadow
2. Shadow Edge
3. Halftone
4. Reflected Light
5. Full Light
Learning light and shade is a very detailed subject. These three books will help you understand the elements of shading (5 values) and the different types of lighting :
1. Cast Drawing Using The Sight-Size Approach by Darren R. Rousar
2. Light, Shade, & Shadow by E.L. Koller
3. Lifelike Drawing With Lee Hammond