You have to find the greatest common factor of the numbers in your fraction. Take the numerator. List all the factors of that number. For example, if your numerator is 48, you would list all the numbers that can go into 48. They would be 1, 48, 2, 24, 3, 16, 4, 12, 6 and 8. Do the same for the denominator. For example, if the denominator is 60 your number list would be 1, 60, 2, 30, 3, 20, 4, 15, 5, 12, 6 and 10.
Look at both lists of numbers. Find the largest number that appears on both lists. In the example of 48/60, many numbers are the same on both lists, but 12 is the largest number that is common to both lists. This number is your greatest common factor.
Divide the numerator of your fraction by the greatest common factor. In the example, the numerator is 48, so divide 48 by 12 to get 4. Do the same with the denominator. In this case, the denominator is 60, so divide that by 12 to get 5.
You are now finished reducing your fraction and your answer is 4/5. It is considered reduced because you can't make the numbers any smaller. No other numbers will go into both 4 and 5, meaning that your fraction is in its simplest form.