You do not legally need your medical certificate in order to begin flight training - you only need it once it's time for you to fly solo. That said, we highly recommend obtaining your medical before starting training, so you don't waste time and money in the case that you unfortunately don't pass the medical.
For your Private Pilot's License, all you need is a 3rd class medical. This is the most "relaxed" out of the three classes. The 2nd class medical is required for commercial pilots, and 1st class is required for airline pilots. Please note that each of these classes have various expiration dates depending on the class and your age, and the class simply downgrades when the expiration period passes.
If you know that your goal is to eventually fly for the airlines, you might consider getting your 1st class immediately when beginning training. Although the medical will expire/downgrade before you get to the airlines, this way you will have peace of mind that if you maintain your current state of health, your health will not prevent you from becoming an airline pilot.
FAA Medical Certificate:
To obtain an FAA medical certificate, you must schedule an appointment with an Aviation Medical Examiner (AME).
To locate an AME near you, the FAA has provided the following link: https://designee.faa.gov/designeeLocator
*Please note that we have found this link to be ineffective at times. We recommend also searching "Aviation Medical Examiner [your ZIP code]" into your browser and looking through the results. Call the AMEs ahead of time to ensure they are able to provide the specific class of medical you wish to obtain.
Local AMEs:
Dr. Efi Kessous, Gaithersburg, MD: https://mmccare.com/urgent-and-primary-care-staff/
Dr. Vinu Ganti, Germantown, MD: https://www.doctorganti.com/
Dr. Tim Oeltmann, Washington, DC: https://www.oeltmannaeromedical.com/
Prior to attending your appointment, you will be prompted by the physician's office to complete a MedXpress form at this link: https://medxpress.faa.gov/MedXpress/Disclaimer.aspx. This form will ask you to report years worth of medical appointments and history in order to have a complete understanding of your health status. Please bring your MedXpress confirmation number with you to your FAA medical appointment.