Who does the responsibility for impeaching someone lie with?

Article I of the U.S. Constitutuion specifies that the House of Representatives "...shall have the sole Power of Impeachment." Article I also states that the Senate "...shall have the sole Power to try all Impeachments." And "When the President of the United States is tried, the Chief Justice shall preside." In other Words, the House of Representatives makes the decision to Impeach the President. It then presents the Articles of Impeachment to the Senate where the case is tried. The Chief Justice of the United States presides over the trial, and the Senators vote on whether or not to convict the President. Conviction requires a 2/3 vote.