What to do if i have been adviced by my cardiologist for a coronary artery bypass graft surgery?

Likely correct: If he thought medicine, or stents would work he would do that. He is seeking a second opinion with a cardiac surgeon since he cannot fix the mechanical situation he has identified. Get the opinion. Understand the risks, benefits, and any alternatives. Likely you will benefit unless you have other significant life threatening issues. Stop smoking, control diabetes, cholesterol, take your meds.

Without knowing : The specifics of your situation, and if this is not an urgent situation, and you don't know what to think or do, to go back to your cardiologist and ask more questions. If still, you don't know, ask for a second opinion or talk to the cardiothoracic surgeon.