Learning shading is the most important part of drawing
1. Get a pencil and some good quality sketching paper. Strathmore is a good brand of sketch books. You can find these at hobby stores.
2. Once you have your pencil and paper, turn the book so it's in landscape and draw a large circle on one half of the page. Make sure there's ample space around it on all sides.
3. Draw a much smaller circle on the other half, towards the upper corner but not in it. This is your light source. Imagine that it is angled slightly in front of the larger circle.
4. Darken the outline of the circle on the side farthest away from the "light source".
5. With a very light hand, draw a circle where the highlight on the larger circle would be. Imagine that the circle in a sphere, and you are shining a flashlight on it from the spot where the "light source" is.
6. Gradually shade in the shadows of the "sphere", getting lighter toward the highlight and darker away from it.
7. Give the sphere dimension and the background depth by drawing a shadow, away from the light source.
Once the sphere looks realistic enough, you can use a stiff paintbrush, a facial tissue that doesn't have lotion, or your fingers to lightly smudge the pencil and give it a softer, less sketchy look.