From Leonardo da Vinci to Superman, we all have been fascinated by the idea of flight and envied those with that ability. If all are equal, then the bird or plane will fly in a level path. The pull of gravity can be thought of as the thrust. Think of a person on a bicycle going faster and faster downhill as an example. Lift is generated when the forward motion through the skies overcomes its weight.
Birds create lift by moving forward.
Think of the times you've seen a tern or gull running into the wind to begin flight. In very simplified terms, the upper air on top of the wing sucks the bird upwards while the increased pressure underneath also pushes it upwards. To prove this to yourself, take a piece of paper and hold two corners, allowing the paper to hang down. Blow hard across the top edge and the paper will rise. Drag is the force that tends to push something along with the air flow.