In this digital painting fundamentals lesson you’ll learn about grayscale painting from reference! Jon shows you how to lay-in an accurate initial drawing and then work with shapes and values. He’ll also show you some ways you can utilize your software to its full potential.
Related Links:
Getting Started with Digital Painting
How to Draw a Figure
Digital Painting Basics - Simple Forms to Complex Paintings
Now that you've seen me do it, it's your turn to practice working from a reference to understand how shapes and values work in real life! 
Start with a clean drawing, do a value block-in, and then build up your painting working from general to specific details.
You're more than welcome to find your own reference, but I've also provided three references that can be found in the downloads tab for this lesson. There is an easy reference, an intermediate reference for those who'd like to try clothing a figure and painting fabric, and a hard reference with lots of costume details and fun material challenges.