Check if there is a GCF between the top and bottom. Such as in 2/4, both 2 and 4 are divisible by 2. So divide both the top and bottom by 2, and you have 1/2. There is not common factor in both 1 and 2 except for 1. Hope that helps! :)
When you can no longer find the a common factor between the numerator and the denominator. This can be done by prime factorization (easy to understand, but laborious), or by Euclid's Algorithm, which...