Check both numbers to see if they have common factors. If they do, divide both numbers by that factor.
Factors for 3: 1, 3
Factors for 12: 1, 2, 3, 4, 6, 12
They both have a factor of three in common, so divide both the numerator and the denominator by 3 to get the simplest form of the fraction.
(3/3)/(12/3) = 1/4