A chemical equation describes what is occurring in a chemical reaction and is made up of two parts, the reactants and the products. To create a chemical equation you need to know what is going into the reaction as that will determine what, and how much, is going to be produced. Sometimes you can combine things together many different ways and produce many different things (Hydrocarbons are a perfect example), so it is also a good idea to know the end result of the reaction too.
Take Hydrogen gas (H2) and Oxygen gas (O2), If you react these together you will make water,
H2 + O2 --> H2O
But there is a catch, water has twice as many Hydrogen atoms in it per molecule as it does Oxygen, so it should require twice as much Hydrogen gas as Oxygen gas. To
balance this equation, we put coefficients infront of certain substances,
2H2 + O2 --> 2H2O, this was there are 4 Hydorgen atoms and 2 Oxygen atoms on each side.
Perhaps this will also be of some help,
http://en.wikipedia.org/wiki/Chemical_equation