Understanding Chemical Formulas:
A chemical formula is a combination of elemental symbols and subscript
numbers
that is  used to show the composition of a compound.   Depending of the
type of compound that the formula represents, the information that it provides will
vary slightly.  

The chemical formula for Sulfuric Acid is shown below.  It tells you exactly how
many of what type of atom is needed to create a molecule of Sulfuric Acid
Chemical Formulas and
Reactions
H2SO

Hydrogen
^
Two
Hydrogen
Sulfur
Oxygen
^
No number
means one
^
Four
Oxygen
So it would look like this.
Two hydrogen (white)
One Sulfur (yellow)
Four Oxygen (Red)
2H2SO4
When we add the coefficient 2 to the front of the chemical formula, it means
we have two of those molecules.  So it would look like the figure below.
Notice now we have to multiply the coefficient by the number
of atoms in each molecule to get the total number of atoms.
Four Hydrogen (white) (
2 x 2)
Two Sulfur (Yellow) (
2 x 1)
Eight Oxygen (White) (
2 x 4)
Using
Coefficients
A chemical equation, like the one shown below, describes how the atoms rearrange to
form different substances.  Notice in the one below, we start with methane and oxygen
and it becomes carbon dioxide and water.  Notice also that the number of atoms of each
type does not change, they are just rearranged.  Sort of like taking legos apart and
putting them together in a different way.
It may seem off the subject, but remember what heat is? (the movement of molecules)  
That is important to remember now.  We think that these reactions happen because of
collisions between the molecules.  The faster the molecules are moving, the more
chance that they will collide with enough energy to form new substances.