Explain the first stages of aerobic respiration that occur before the Krebs cycle.

Aerobic respiration is a controlled release of energy from organic compounds to produce ATP with the use of oxygen. It occurs in several stages:
Glycolysis starts with phosphorylation (addition of phosphate molecule) to glucose. Phosphorylated 6C sugar molecule is less stable, hence it is broken down into two 3C sugar molecules. They undergo oxidation-removal of hydrogen atoms. Energy released by oxidation is used to convert ADP to ATP. 2 pyruvate molecules are formed as a result from one glucose molecules. 
Link reaction link glycolysis with Krebs cycle. If oxygen is available pyruvate enters the mitochondrion, where oxidative decarboxylation of pyruvate takes place. Oxidative, because hydrogen atoms are removed and decarboxylation, because of removal of CO_2. As a result acetyl group is formed. It is attached to coenzyme A creating acetyl coenzyme A.