I would like to become a ranger of the ncr, offiicially, you what I mean?
Can you please tell if thats possible and where I must to become one

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.

Kill the slavers...oh wait thats FO2!
http://images1.wikia.nocookie.net/__cb20100220224056/fallout/images/2/23/FO2_Ranger_Pin.gif

LOOOOOOOL

It's not possible. Kind of svcks really but it would ruin the whole concept of doing whatever you want because the NCR would just draft you to some camp waiting for legion hit squads that never come.

Exactly.

You don't have to kill a Veteran Ranger to get the armor. It's on the shelf at the NCR Safe House. Just do things for them and after you've done enough, they'll give you a key to the Safe House.

You can't officially all the NCR will ever give you is a Golden Branch: Their highest most trivial award (Bar the reward for hardcoe Mode)

If you want to Roleplay you could follow Patrols and try and make everybody under their rule.