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If you are unable to balance an equation by inspection, use a structured method instead. Let's balance the reaction between blood sugar (glucose, C6H12O6) and oxygen in a structured way. This reaction, called cellular respiration, provides the internal energy for all living animals, including you, and produces carbon dioxide (CO2) and water (H2O). 
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There are 3 different elements to balance in the equation: C, O and H. All 4 substances involved in the reaction are made up of different combinations of these 3 elements.
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An effective method to balancing chemical equations is to begin with the elements that you see in the fewest compounds and end with the element that you see in the most substances.
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The respiration reaction shows both hydrogen and carbon are involved in only one reactant and one product. You could start with either carbon or hydrogen, so let's choose hydrogen. A coefficient of 6 in front of the H2O product will balance the number of hydrogen atoms. Note that adding this coefficient also changes the total amount of oxygen on the product side from 3 to 8 moles.
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Let's move on to carbon next. You can balance carbon by placing a coefficient of 6 in front of the CO2 product. Inserting this coefficient changes the amount of oxygen on the product side from 8 moles to 18 moles.
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