I need to know how to get the m1911 pistol

The M1911 is the BF3 veteran weapon so you must have a Battlefield veteran status of 1 or greater to access it.  That means that you've played BF3 and at least one other Battlefield game while logged into your Origin account.  You can get veteran status very cheaply by buying a used copy of Battlefield Bad Company 1 or Bad Company 2 which should all be very inexpensive in the used section, or you can play one of the free web based Battlefield titles such as Battlefield Heroes http://www.battlefieldheroes.com/en/ or Battlefield Play 4 Free http://battlefield.play4free.com/en/ while logged in with your Origin account.  These two titles will give you veteran status as well.