Don't know if still the case, but in order to become a 'Registered Alien' you must be sponsored by an immediate family member, (Parent or sibling), who is US Citizen, or by a Spouse who is US Citizen, or by a US based employer who needs your specific skills set, which can be found on the INS approved list.
After 5 years of being a fine, upstanding, law abiding and financially solvent 'guest' of Uncle Sam, you can then apply for US Citizenship.
Federal agencies only employ US Citizens, the exception is/was the military, who may still accept Registered Aliens from specific countries. (Those which are NATO member states for example).
Been a while since I lived there so all/some of the above may have changed.