The requirements to impeach a president are found in Article II, Section IV of the Constitution. It states that the President, Vice President and all civil Officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery or other high crimes and misdemeanors.
The impeachment process begins in the House of Representatives where the House Judiciary Committee decides whether or not to even proceed with impeachment. Provided they do, the Chairman of the Committee proposes a resolution calling for a formal inquiry into the issue of impeachment.
Once the inquiry is complete, a resolution is sent to the full House declaring whether impeachment is warranted or not. The Full House debates and votes on each article of impeachment, and if any one of the them is approved by a majority vote, the President is then considered to be impeached. The process then moves to the Senate where the Articles of Impeachment are sent from the House. A trial is then held, pursuant to rules and procedures set up by the Senate.
In the trial, various House members serve as prosecutors, and the President, as the defendant, is entitled to be represented a lawyer. The Chief Justice of the Supreme Court presides over the proceedings, and the full Senate acts as the jury. Once the evidence has been presented, the Senate debates a verdict, which requires a two-thirds vote for a conviction. If the President is convicted, the Senate can vote for removal from office.