Start out with a visit to a doctor called a urologist. He'll give you a physical exam and ask you questions about your lifestyle and medical history, such as:
Surgeries you've had
Medications you take
Your exercise habits
Whether you smoke or take recreational drugs
He may also have a frank discussion with you about your sex life, including any problems you've had or whether you have or ever had any STDs (sexually transmitted diseases). You'll probably be asked to give a sample of semen for analysis.