In cullular respiration, there are four stages (this is how I learned it when glucose is being broken down). The stages are glycolysis, formation of acetyl coenzyme A, krebs cycle, and electron transport system. -Formation of acetyl coenzyme A is also sometimes called oxidation of pyruvic acid.-
In glycolysis, glucose is broken down to make two molecules of pyruvic acid. Four hydrogen atoms are also produced (and 4 ATP [a net gain of 2 ATP]). Please note that when I said "produced," they're either being taken from the present molecules or added.
In the formation of acetyl coenzyme A, the pyruvic acid molecules are used along with 2 molecules of coenzyme A. The end product of this stage is two molecules of acetyl coenzyme A, two hydrogen atoms, and two carbon dioxide molecules.
In the krebs cycle, two molecules of acetyl coenzyme A are reacted with three oxygen molecules. Which brings the products of six hydrogen atoms, four carbon dioxide molecules, and, again, two molecules of coenzyme A (and around 4 ATP).
Finally, in the electron transport system, twelve hydrogen atoms are reacted with three oxygen molecules. This brings the product of six water molecules (and 36 ATP).
Take note that this is just one molecule of glucose being broken down for respiration.