Explore the lesson "Elements of art", offered by Khan Academy, paying attention to the different concepts addressed:

• Line
• Shape and form
• Color
  • Do these interactive activities on color-related adjectives.
  • Answer these questions on basic color theory.
• Space. The video Surface and Depth, also contained within this unit, can be very helpful.
• Texture

It may also be a good idea to take a look at the video Light and Shadow.