Choose a reference frame. Orient the X & Y axes. (Most often X is chosen in the horizontal direction and Y is chosen in the vertical direction.)
Choose a convenient point to calculate moments around.
Apply the 3 equilibrium equations and solve for the unknowns.
Two children balance a see-saw in horizontal equilibrium. One
weighs 80 pounds, and the other weighs 60 pounds and is sitting 4 ft. from the
fulcrum. Find the force the fulcrum applies to the beam and the distance to the
fulcrum to the 80 lb. child. (Neglect the mass of the see-saw.)