You will need to catch at least 60 different Pokemon and defeating the Elite Four, then talk to Prof. Oak in Pallet Town.
You have to trade with firered or leafgreen to get the national pokedex.
see every Pokemon
beat the Pokemon league.
The national is the National Pokedex. This pokedex allows you to catch/see new pokemon. You'll also get to go to 4,5,6, and 7 island.
You have to own 60 Pokemon to get the national pokedex.
get firered or leafgreen and migrate it to pearl after national pokedex
YOU NEED TO FIGHT ALL POKEMON AND YOU WILL COMPLETE THE POKEDEX. AND THAT WILL HELP YOU TO GET THE NATIONAL POKEDEX BY TALKIN TO PROFESSOR OAK.
see every pokemon in the kanto region then visit professer oak
A national pokedex is usually gained late game, the national pokedex is an upgraded pokedex which records information about Pokemon around the nation, including Pokemon from previous generations, a national pokedex also allows Pokemon to evolve into forms which have been discovered in different games or generations i.e (in Pokemon Firered and Leafgreen, it is only possible to evolve golbat into crobat after acquisition of the national pokedex).
trade it from ruby sapphire or emerald after getting the national pokedex.
you find in snow point temple after you get the national pokedex
You go to route 228 after you get the national pokedex and migrate Pokemon from leafgreen