The first step should be writing out a balanced equation. To do that, you should remember that ethanol is a hydrocarbon derivative and burns the same ways as hydrocarbons yielding H20 and CO2 as end products. With that in mind, here is a balanced equation for the reaction:
C2H5OH + 3O2 –> 2CO2 + 3H2O
Now, you need to find out which of these compounds is the limiting compound. To do this, you must understand which element that will be used up first. To do this, compare the value ratio of ethanol to oxygen that you have. From the equation, you need just 1 part ethanol to 3 parts of oxygen for the equation to take place. Suppose you have one of these exceeding the value it should have, then the other one is the limiting reactant. For instance, if you have an ethanol to oxygen ratio of 1:3.5, as against 1:3, oxygen becomes excess and now ethanol becomes a limiting reactant. The limiting reactant will be used up completely while there would be leftover oxygen.