I went to an online symptom checker. It said I might have hypertrophic obstructive cardiomyopathy. What do doctors say are the most common symptoms?

Really? That's a tricky diagnosis for any entity that has not seen or examined you. The symptoms of hocm are quite common (breathlessness, chest discomfort, palpitations) as are the signs. If hocm is even a remote possibility, you best be seen by a cardiologist to confirm (or more likely, deny) the diagnosis by physical examination and an echocardiogram with/without physiologic maneuvers (he'll know!).
Variable. Some patients have no symptoms. The first symptom may be sudden collapse and possible death. Others include chest pain, fainting, palpitations, high blood pressure, heart failure, shortness of breath, easy fatigue, exercise intolerance.