Look at both the numerator and the denominator. If you can find a number that divides evenly into both, divide that number out of both the numerator and the denominator.
You now have a reduced fraction. Now, go back and do the previous step again and again, until you can't do it anymore.
For example: 210 / 378
Since they are both even, 2 will divide into both the numerator and the denominator; after dividing each by 2:
= 105 / 189
They aren't even, so 2 won't work again, try 3:
= 35 / 63
If you try 3 again, it won't work (35 isn't divisible by 3); 5 won't work (63 isn't divisible by 5), but 7 will:
= 5 / 9.
No more numbers work; you're done!