You cannot convert cubic yards to tons directly because the two units measure different things. Cubic yards measure volume, but tons measure weight. Converting one to the other makes little more sense than converting inches to miles per hour. Nonetheless, you can convert between unconnected units if you know the factor that links them. With volume and mass, the factor is density--mass per unit volume. Researchers have documented dozens of substance's densities.
Multiply volume by "27" to convert to cubic feet. For instance, if you have 10 cubic yards of concrete:
10 * 27
\= 270 cubic feet
Multiply your answer by density in pounds per cubic foot. Concrete weighs 150 lbs. per cubic foot.
270 * 150
\= 40,500 lbs.
Divide your answer by 2,000 to convert to tons.
40,500 / 2,000
\= 20.25 tons