The President, What does he do?
As described by Article 2 of the Constitution, the President has roles and powers which include: Commander-in-Chief, Chief Executive, Head of State, Chief Diplomat, and Legislator. As Commander-in-Chief the President controls the US Military. As Chief Executive, the President is sworn to uphold and defend the Constitution's laws, as well as to make sure those laws are executed. As Head of State, the President is the cerimonial leader of the US. As Chief Diplomat, the President meets with foreign diplomats, appoints ambassadors, and fulfills the obligations to negotiate treaties, agreements, and understandings with foreign powers. As Legislator, the President is required to present a State of the Union address to Congress each year.
The President's Cabinet
The President's Cabinet is essentially an advisory board. They have no power beyond what the President allow them and are only members to advise the President on matters pertinent to their respective departments.
Requirements for Office
So what does it take to be President? That is answered in Article 2, Section 1 of the Constitution and there aren't many requirements. You just have to be a natural born citizen and 35 years old. That's it.
Presidential Line of Succession
Presidential succession is lined out in Amendment 25. If the President dies or is otherwise unable to fulfill the duties of office, then there is a plan for who will take over as President. It goes in this order: Vice President, Speaker of the House, President Pro Tempore, Secretary of State, Secretary of the Treasury, Secretary of Defense, Attorney General, Secretary of the Interior, Secretary of Agriculture, Secretary of Labor, Secretary of Health and Human Services, Secretary of Housing and Urban Development, Secretary of Transportation, Secretary of Energy, Secretary of Education, Secretary of Veterans Affairs, and finally the Secretary of Homeland Security.