If your hips rock from side to side when you pedal, it's too high. Toe clips, metal cages that help keep your feet in contact with the pedal, can save your knees by allowing you to lift up as well as push down when you're turning the cranks. It'll take a couple of outings to get used to the clips, but after that you won't feel comfortable riding without them. Eventually, if you want to extend your pastime into doing long weekend rides, you may want to get clipless pedals, which call for cycling shoes that snap into place. Wearing one reduces the risk of serious head injury by more than 85 percent.
Fluidity is key in cycling.
Stay in a gear that allows you to spin the pedals at about 70 to 80 revolutions a minute. It may seem too fast at first, but you'll quickly adjust. This cadence will get you through valleys and over hills efficiently, and it will help prevent stressed joints and strained muscles. If you remember to shift down anytime your cadence slows, you'll do fine. Only the foolhardy pick high gears to show how tough they are.