Well first he both needs to be successfully impeached by the House, then the Senate has to approve the impeachment by a 2/3rd vote(which acts as a conviction and forces him out while also ensuring he is barred from ever holding a public office position again).
No president has yet ever reached a full impeachment conviction(though both Andrew Johnson and Bill Clinton were impeached by the House, but were not convicted by the Senate and thus were able to maintain their position). Nixon resigned to avoid the expected impeachment trials.
Keep in mind also that Republicans do have majority, so this could be a LONG process, one in which is still an uphill battle until enough mounting evidence ensures the R party turns hard on him(they are a bunch of selfish pricks, so there is hope).
If and when that happens, and assuming Pence is not liable and still in office. It would go to him likely. Depending on how crazy this thing turns out, who knows though.