A free-body diagram is a sketch showing an object and all of the forces being exerted on it. Always draw a free-body diagram when you are analyzing forces.
Consider some basic examples.
A box is at rest on a flat table. Which free-body diagram is correct?
A box is moving at constant velocity to the right. Which free-body diagram is correct?
In both cases above, mg and N are equal in magnitude and opposite in direction. Are these the equal-and-opposite forces discussed in Newton's third law?