If your weight is force created by gravity on your body, and therefore is the Earth pulling you down, what is the reaction force?

The reaction force is the force exerted by your body on the earth pulling upwards.

Explanation:

This can be explained using Newton's third law of motion which states that if object A exerts a force on object B, object B exerts a force of equal magnitude and opposite direction on object A. One characteristics of these force are that they act o 2 different bodies. In this example, the earth exerts a force on you. The Newton's reaction force would be you exerting a force of equal magnitude on the earth.