How is pyruvate converted to acetyl coA?
Pyruvate is converted into Acetyl CoA in an intermediate process just before the Citric Acid Cycle.
Here it reacts with Coenzyme A. Here it loses two of it's oxygens and one of it's carbons to form Carbon Dioxide. Also, one molecule of NAD+ is reduced to form NADH. The resulting molecule is a molecule of Acetyl CoA.
This diagram I drew should help sum everything up:
In addition, here's a video that explains this, in case you like it better that way:
Hope that helped :)