https://litigation.findlaw.com/legal-system/presidential-impeachment-the-legal-standard-and-procedure.html
Both the U.S. House of Representatives and the U.S. Senate have the right to make their own rules governing their procedure, and to change those rules. Under current rules, the actual impeachment inquiry begins in the Judiciary Committee of the House of Representatives. That Committee holds hearings, takes evidence, and hears testimony of witnesses concerning matters relevant to the inquiry. Typically, as occurred in the case of President Nixon, there will also be a Minority Counsel who serves the interest of the party not controlling Congress.
Witnesses are interrogated by the Committee Counsel, the Minority Counsel, and each of the members of the House Judiciary Committee. The Committee formulates Articles of Impeachment which could contain multiple counts. The Committee votes on the Articles of Impeachment and the results of the vote are reported to the House as a whole. The matter is then referred to the whole House which debates the matter and votes on the Articles of Impeachment, which may or may not be changed. If the Articles of Impeachment are approved, the matter is sent to the Senate for trial.