Octane is C8H18, Carbon Dioxide is CO2, Oxygen is O2, and water is H2O. Thus, the reaction will be in the form:
C8H18+O2 -> CO2+H2O
However, it is not balanced. We start with just one C8H18. This requires 8 CO2s to account for the 8 carbon atoms and 9 H2Os to account for the 18 hydrogen atoms. This is a total of 8*2+9=25 oxygen atoms. We can't have 25/2 of a molecule, so we multiply all coefficients by two. Therefore, our balanced equation is:
2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O
