Officialy I don't think you can, at later events during the MQ you can add elite ranger armour by killing one.
You can help the NCR out with all their quests, but I have yet to be knighted as one unlike minor factions.
NPC's will however respond to your rep as if you're part of the NCR force.
Rangers themselves are not officially ranked members of the military apart from a few leaders that are ex vet's of the rangers.
Mostly they're a milita given extra training recruited primarily from within the NCR, and its military.
But anyone with a gun and iniative could name hemselves a ranger, as long as you don't act against NV or NCR.