For this achievement you have to solve the NCR's problems:
After completing the first two NCR quests, (Things That Go Boom and Kings' Gambit), head to Hoover Dam and talk to Colonel Moore.
You will now have to deal with the Great Khans, the Omertas, Mr. House and the Brotherhood of Steel. Your last objective will be to protect President Kimball.
Since this is Fallout, there is more than one way to skin a cat. So I recommend to use this very helpful and detailed walkthrough:http://fallout.wikia.com/wiki/For_the_Republic