To become a private pilot you must first be a student pilot. To become a student pilot and start your pilot training you must complete the following: present both a (birth certificate & State I.D.)
 
Then, to fly an airplane solo (alone in the aircraft) you must complete the following:

You must be at least 16 years of age to become a student pilot

Be able to read, speak, write, and understand the English language. If the applicant is unable to meet one of these requirements due to medical reasons, then the Administrator may place such operating limitations on that applicant's pilot certificate as are necessary for the safe operation of the aircraft.

You must pass at least a third-class medical exam.  (Not Hard)

You must receive an endorsement from an authorized instructor authorizing you to fly solo.

If you are not an American citizen there is some paperwork needed.
 
Finally, to command an aircraft alone (solo) you must have non-owner insurance.