Apparently, some people slept though Civics/U.S. Government class. Apparently, some people missed out on the impeachment of President William J. Clinton and the attempted impeachments of Presidents Richard M. Nixon, George W. Bush and Barack Obama.
So let's take it by the numbers:
0. An impeachment is not a conviction.
An "impeachment" is precisely analogous to an "indictment." Neither indicates guilt, only that sufficient evidence has been presented to justify a trial. President Andrew Johnson was straight-up impeached, missed being found guilty by one (1) vote, and served the remainder of his term.
1. What constitutes an impeachable offense?
The U. S. Constitution says "high crimes and misdemeanors." A "high crime" is one that you are only enabled to commit by being an office holder -- a Federal judge who takes a bribe to throw a case has committed an impeachable offense, but if you slip a bum a five dollar bill to commit perjury for you, that's just a regular crime. A "misdemeanor?" It really is what it sounds like, a petty crime, though presumably also modified by "high." Yes, one might attempt to impeach a President for jaywalking, especially if he had the Secret Service stop traffic. The nature of possible offenses is broad and as a practical matter,
an impeachable offense is anything the House of Representatives says it is. Don't like it? Take it up with the men who wrote the Constitution.
2. Yes, the impeachment process can (and normally does) start in a House committee.
They're not going to tie up the whole House until they think they've got something they can get the House to vote for -- besides, Representatives get better screen time in committee.
3. Yes, Congress has subpoena powers.
Even over trivial nonsense, House, Senate and committees thereof can haul you in and have you answer questions under oath. It turns out that "Contempt of Congress" is an actual crime, unlike holding Congresspersons in contempt. There are all sorts of interesting complications where this intersects Executive Privilege and National Security matters, which will no doubt make for fascinating scholarly papers, but the basic principle isn't in any doubt.
4. If a President is impeached, there is a trial.
The trial is held in the Senate, with the Chief Justice of the Supreme Court presiding. Verdict is determined by vote and it takes a two-thirds majority to convict. Conviction results in removal from office, period. Nobody goes directly from the White House to the Big House. (For Federal officials other than the President, the President of the Senate presides. You may know him as the Vice-President of the United States; presumably, a Vice-President who'd been impeached would recuse himself and let the President Pro Tem preside.)
5. Succession is clear.
Remove the President, the Vice-President steps up. That's how it works. There's no ambiguity to it; Congress doesn't get a "twofer" and a Vice-President who succeeds a President gets to name his own Vice-President.
So, Republicans who are fuming that the present hearings are a "coup," Democrats salivating over how they'll shove out the Chief Executive in a trice? Yeah, no. To both groups. This is a thing that will take its due course. It's something the Federal government does pretty regularly these days. It will play out and you can either watch and learn, or spend all your time spun into a web of fantasy.