Understanding the basics of highlights and shading is important.  If you get these basics fundamentally wrong, it can make the entire work look unbalanced.  Understanding lighting and where the light is coming from and how that depicts the shadows on or around the object, will help you make your artwork shine.

Refer links below, that cover the basics of highlighting and shading

http://willkempartschool.com/how-to-acrylic-paint-light-and-shade-part-3/

http://design.tutsplus.com/articles/improve-your-artwork-by-learning-to-see-light-and-shadow–cms-20282

http://www.wroberts.com.au/html/about_painting_light.html