how do i get to be an ncr ranger and buy ncr armour??

and buy guns from the ncr??

You can't become an ncr ranger, you can only side with them. You can however pick up their armor from their dead soldiers though.

If you help the NCR, you'll receive the key to the NCR Safe House from Colonel Hsu. The armor is on a shelf there along with some weapons; you don't have to kill a Veteran Ranger. There are also weapon suppliers you'll be able to buy from. Don't forget to visit Gun Runners too.