External imagery is like watching yourself on television. You are looking at yourself from somewhere outside your body. Internal imagery is where you're looking out at the world through your own eyes. With this imagery you sense your body doing things, think through how specific motions feel. Like any skill, imagery must be practiced.
Some people find it easier to do than others, but anyone can learn to do it.
It is best to start visualizing with simple, familiar objects. For example, get an orange. Take a moment to look at it, feel it, and smell it. Now put it down and close your eyes. Try to picture the exact shape and size of the orange.