Well, you basically need to figure out how many copies of each molecule there are on each side such that there are the same numbers of each kind of atom on each side of the equation. Here's an example:
Balance this equation:
C4H10 + O2 => CO2 + H2O
This is a basic combustion reaction. You'll probably see something similar at some point if you haven't already. Count up the atoms on both sides:
Left: Cx4, Hx10, Ox2
Right: Cx1, Hx2, Ox(2+1=3)
Well, to start with, the carbons aren't the same. You know that the same number of carbons should be on both sides, so try multiplying the carbon containing molecule on the right by 4.
C4H10 + O2 => 4 CO2 + H2O
Left: Cx4, Hx10, Ox2
Right: Cx4, Hx2, Ox(8+1=9)
Now, can we multiply the H's on the right without messing up the C's? Yes we can. We multiply the water molecules by 5.
C4H10 + O2 => 4 CO2 + 5 H2O
Left: Cx4, Hx10, Ox2
Right: Cx4, Hx10, Ox(8+5=13)
Now all we have to do is balance the oxygens. However, you can't multiply O2 by anything to get 13. So, try doubling everything so that the 13 becomes an even number and you can find a multiple of O2 that will work.
2 C4H10 + 2 O2 => 8 CO2 + 10 H2O
Left: Cx8, Hx20, Ox4
Right: Cx8, Hx20, Ox(16+10=26)
How many more O2's do you need to make 26? Well, you need 26 total O's, so you need 13 total O2's, and you have 2...so you need 11 more O2's for 13 total.
2 C4H10 + 13 O2 => 8 CO2 + 10 H20
Left: Cx8, Hx20, Ox26
Right: Cx8, Hx20, Ox(16+10=26)
And the equation is balanced.
It's just a process where you make sure the number of atoms on each side is the same.
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