Pilots are required to take an FAA medical examination. There are three classes of medicals- 1st, 2nd, and 3rd. I would recommend that initially you get a 1st class medical (which is the most strict and required for flying at the airlines), to ensure that no surprises come up.

Take a look at the FAQs about medicals on the FAA website, then if you have any specific questions, post them back here.

http://www.faa.gov/licenses_certific...ification/faq/

Additional information about what is covered in each medical can be found here:
http://ecfr.gpoaccess.gov/cgi/t/text...67_main_02.tpl

As for diet, as long as you maintain a healthy lifestyle you should do okay. Certainly exercise is highly recommended, since the job mostly consists of eating bad food (fast food), and sitting for long periods of time.