I more or less answered that question here two days ago:

What was Nixon's public life like after his resignation?

In short, immediately after his resignation, he was on an allowance, that quickly ran out. He managed to regain financial stability by doing the Robert Frost interviews ($600.000) and his first memoirs ($2.3 million). For the rest of his life he mostly worked as an author on books on foreign policy, with the last one being completed just two weeks before his death, along with public speaking engagements on the same topic. He was marginally involved with politics, mostly for Reagan, and mostly in an unofficial capacity.

Oh, and a pretty important detail, Nixon was never impeached. He resigned before he could be impeached, then was pardoned by Gerald Ford.