[JugglingReferee]

NFHS is for high school basketball in the United States, and currently also in Ontario, Canada.

NCAA is for college/univeristy basketball in the United States.

FIBA is the rule set for international basketball, such as found in the Olympics. FIBA is also used in many other cases: such as youth basketball across Canada, and in high schools in Canada, but outside of Ontario.

You're probably in the US, so the best rule set for you to study, at first, is Federation (NFHS). If you're very successful with officiating, you may enjoy hitting the international circuit, such as being an official for the PanAm games, the Olypics, the World University Games, the World Asian Games, etc.... in which case you'll need to be FIBA certified. One NBA official, Scott somebody, is actually FIBA certified.

Other leagues can use whatever rule set they want. For example: men's leagues in your area, though not part of the school system, can use either Federation or NCAA rules. Or they can use FIBA rules or even NBA rules.