To simply this, you just divide the numerator and denominator by its GCF (Greatest common factor)
Factors of 18 : 1,2,3,6,9,18
Factors of 30 : 1,2,3,5,6,10,15,30
As you can see, the great common factors of 18 and 30 is 6.
So divide 18 by 6 and 30 by 6, and you get 3 and 5
Therefore, the simplest form is 3/5