To answer this question we need to remember Newton's law:
First law: if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force.
Second law: a force applied on a body will alter its movement.
Third law: when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.
Now that we know this let's think on the takeoff of the plane.
At first the air around the plane will with some kind of motion (does not really matter which as long as the conditions are safe to make the takeoff) once the plane start to move the wing will exert a force on the air, changing its direction (downward) and velocity, according to Newton's second law. Since the wing is exerting a force on the air, by Newton's third law, the air will exert a foce on the wing that is equal in magnitude but have opposite direction (upwards) this force is called lift. This is the force responsible for making the plane takeoff.