Ok, first things first. When hooping around the waist you want to be standing up straight with feet shoulder width apart. Hold the hoop against the small of your back so that when you push it around your body it will start spinning around your waist, or the thinnest part of your torso. To give the hoop the best chance of staying up, you want to give it a nice steady push so that the hoop is as horizontal or flat as possible.
Then, imagine that you have a large pencil extending from your nether regions down to the ground, and move your hips in a circular motion as if you were drawing a circle on the ground. Some people find it easier putting one foot in front of the other and moving their hips in a thrusting motion. It works best when you simply don't think about it at all...just let your body get into a rhythm and go with the flow! Find what works for you and eventually, with practice, you can sharpen your skills to have your feet together (looks nicer), and keep the hoop going with smaller, more graceful movements of your hips.
I practiced hooping around my waist for over 6 months before moving onto other hoop tricks. A good foundation will help you with more advanced moves.