The doctor is an expert on medical issues... you are an expert on you. Before you go to a doctor, please get organized. What symptoms do you have? For how long? How frequently? Write down all the things you want to tell your doctor. Write down all the specific questions you want to ask your doctor. You have one shot at giving the doctor all the information he needs to know about you and your condition. If he has all of the details, he has a better chance at helping you.
For more information see this article by the Joint Commission.