This pose is the gateway to all arm balancing postures.
The most important thing about this pose is also the place where most people commonly go wrong. The wrist in the bottom arm--it should be FORWARD of the shoulder, not right underneath.
This will make you use muscles like serratus anterior to hold the pose, reduce wrist pain, and provide stability. And then, once you've mastered this simple form, it's time to start adding variations!
One step at a time though! Happy (Side) Planking!