As part of the long list of requirementsyou will need to obtain a medical from an FAA approved doctor. It is not required to start your lessons but is required to act as pilot in command (for students that means to fly solo). It is a very good idea to get near the beginning of your training for two reasons 1) the back side is your student pilot certificate and 2) you can rule out any medical issues that would not allow you to pursue your license.
The third class medical is all you will need if you are planning to stop at the private license. There are second and first class for more advanced pilot licenses.
The below website allows you search for your local doctor.
http://www.faa.gov/pilots/amelocator/
Fly at West Valley Flying Club
