Sounds to me like your just starting out, and are getting on the back wheel but keeping the rear brake locked whilst your on it. The easiest way to combat this is to do some very small gapping hops, only about a foot or so, to learn how to inch forwards slowly. Once you can competantly hop a couple of inches forwards at a time using the pedal hop technique (get on the back wheel, let go of brake, stamp on pedals, jump in the air, re apply brake, land) you'll start to be able to shift the wheel along without letting go of the brake. At all times keep your weight forwards more though... ideally you want to be hopping on the spot without moving anywhere, and if you going backwards... it means your too far back. Lean forwards more and keep your elbows bent and you'll be fine.