To balance a chemical equation all you need to do is check that the number of moles for any element on the reactant side is equal to the number of moles of the same element on the product side. Examples, 

CH₄ + O₂ → CO₂ + H₂O

Thus is an unbalanced equation because if you see carefully, 

The number of oxygen atoms on the reactant side is 2, but on the peoduct side it is 3,

And, the number of hydrogen atoms on the reactant side is 4, but on the product side it is 2.

Therefore, we need to now balance the reaction in such a way that all the atoms on the reactant side and the product sides are equal individually.

So, if you see, CH₄ + 2O₂ → CO₂ + 2H₂O

Now if you calculate, the atoms of hydrogen and oxygen on both the reactant and the product sides are equal.

Thus we have balanced the equation. This is how you need to balance all chemical equations.