How to join the NCR in Fallout New Vegas?

Play the main storyline until you reach New Vegas and get to Mr. House's Casino. If you exit the house, you'll get approached by an NCR soldier. he'll send you to the embassy, where the NCR storyline begins.

source: http://www.gamefaqs.com/boards/959557-fallout-new-vegas/56897584

Requirements: Do not murder NCR troops (duh). Doesn't matter if you murder the legion, that's irrelevant. Doing quests for the legion or the Powder Gangers will make the NCR unhappy. Try not to get too far into the main quest, else you might miss your chance.

After the quest "Ring-A-Ding-Ding," you will be approached by an NCR soldier. They will ask you to go to the NCR embassy, where you can begin helping. Side note: being friends with a group called the Boomers will make this quest bug out a bit. If you don't know who they are, that's fine. Don't go walking through minefields.

To build reputation, you can go to Camp McCarran, Sloan, and Camp Forlorn Hope for related quests.

First of all the NCR MUST come to you. After exiting the Lucky 38 after complete the quest "Ring-a-Ding-Ding" After that go to Camp Maclaron and talk to the Bounty hunter and ask where he got the Caps and he will say about the Major. After doing so talk to the Major and ask if you can do a bounty. Choose from the three but be careful about the last choice of the three save that one for last (My personal choice) because of the difficulty. First and the most easist and you can find the second bounty but at the same time it is second to most difficult. After that the last bounty you can have the sniper squaddren to help you kill the target and you get two or five rewards (depending if you want the weapons dropped by the other two bounties) The first reward for helping the NCR is a two way radio and the second big reward is the weapon of the third bounty which is a unique weapon if you want to use a melee weapon. But DON'T KILL ANY NCR SOLDIERS!