Question

How to balance an equation?

Answer 1

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 

Answer 2

To balance an equation, you do whatever you did on one side, on the other side. If you subtract, then you subtract on the other side. If you multiply, you multiply on the other side, etc...