Just as forces cause acceleration and hence changes in translational motion, torques cause changes in rotational motion. If an object is at rest, a net torque exerted on it will cause it to rotate. When you sit on a see-saw, you exert a torque on it and the see-saw will rotates about the center.
To calculate the torque on a body a point, you need to know the magnitude and direction of the force and the distance from the application point of the force to that point.
t = F*d^
When more than one force is involved, the torque is calculated for each separately and then they are added together taking care to include the proper sign.