Newton's third law:
"When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body."
Popularly paraphrased: "For every action, there is an equal and opposite reaction."
There is no "antigravity" that opposes gravity, nor is there required to be such a force to fulfill Newton's third law. When the earth's gravity pulls on you with a certain force, you are also exerting the exact same gravitational force on the earth. That is the "equal in magnitude and opposite in direction" force that Newton's law says exists. The earth pulls on you with 150 pounds, you pull back on the earth with 150 pounds. Same force, opposite direction.