Not too long ago, Lifehacker and Matt Kohr published an excellent guide to Digital Painting for beginners! If you're interested in learning in how to paint or looking how to improve your skills, look no farther! The entire series teaches you the basics, from brushes to erasers! Below is entire series, ready to watch!
(also see Photoshop 101)
Digital Painting 101, Lesson 1: Get Started Painting on Your Computer Text Version
Digital Painting 101, Lesson 2: Brushes and Erasers Text Version
Digital Painting 101, Lesson 3: Mixing Paint