you trade and catch all of the pokemon

you call prof birch and then you go to his lab and he will let you choose from 3 differnt
pokemon

You can get the Johto staters.Here are their evolutions.
Gr***-Chikorita,Bayleef,Meganium
Fire-Cyndaquil,Quilava,Typlosion
Water-Totodile,Croconaw,Feraligatr

You must trade with leaf green, fire red, sapphire, and ruby to get the pokemon you
couldnt then talk to professor birch. Now you can only choose ONE johto starter. If you
want to migrate them to diamond/pearl, you'll have to START OVER and the other two unless
you have friends with the pokemon you dont have and thats willing to give it away.