Fallout: New Vegas/Things That go Boom

After you meet with Mr. House and exit the Lucky 38, an NCR trooper will give you a message to head to the embassy and visit Ambassador Dennis Crocker.

In the embassy, enter the door on the left, and head to the end. The ambassador will tell you the mission, to speak with the Boomers and get their support for the upcoming battle - resetting NCR infamy when you accept the quest (or if you already met the Boomers, when you report the situation). The specific steps for obtaining their support is found in Volare!.

Once you obtained their support, return to the Ambassador to complete the quest.