Do you mean a chemical equation? If so you balance it by adding coefficients. (The subscripts shouldn't change, nor the products or reactants)
Let's say you have (reactant side) H2 + O2 ----> H2O (product side)
To balance the equation count the atoms in products and reactants.
There are 2 hydrogen atoms on both sides, so they are balanced, but the oxygen atoms aren't. To make it balanced, we can add a 2 in front of H2O
H2 + O2-------> 2H2O (4 hydrogen atoms, 2 oxygen atoms).
Now we need to adjust the reactant side since there are 4 hydrogen on the product side. We can do this by adding a 4 in front of the H2:
2H2+O2-----> 2H2O
This is a balanced eqation: reactant side: 4 hydrogen, 2 oxygen
product side: 4 hydrogen, 2 oxygen
