You have to find a common number divisible by both numbers of the fraction until you cannot divide the numbers of the fraction any more. So on your problem 18/42 start by dividing both numbers by say 2
18 divided by 2 gives you 9
42 divided by to gives you 21
Then try dividing them again say by 3
9 divided by 3 gives you 3
21 divided by 3 gives you 7
If you try dividing again you will see that you can no longer divide it so 3/7 would be your answer. I hope that helps.