In order to balance, you want the same number of atoms of each element, on each side of the equation. An example of an incorrectly balanced equation would be: H2 + O2 ---> H2O
As you can see, the number of atoms is not equal on both sides of the equation, therefore you've gotta change the number of atoms. In the formation of water this is relatively simple:
2H2 + O2 ---> 2H2O
That's ideally how you'd like to balance.