To reduce fractions to lowest terms, find the biggest number that divides evenly into both the numerator and the denominator. Then divide and your new fraction should be in lowest terms if you used the biggest number. This number is called the Greatest Common Factor of the numerator and denominator.
Example:
5/10 reduces to 1/2 because 5 divides into 5 once and into 10 twice.
12/15 reduces to 4/5 because 3 divides into 12 four times, and into 15 five times.