You don't. You may only choose one of them.
Not all Pokemon regions are available on all games. SoulSilver only has access to Kanto and Johto.
All you have to do is get to Viridian City.
all i know is that mew is in a cave in kanto
you can find them in either Kanto, Johto or by trading Pokemon with other people
you have to go to every gym in kanto a beat all the gym leaders
To get to Mt Silver you have to defeat the Pokemon league and all of kanto's Gyms.
you get them at the begining of the game by New Bark Town and all through kanto
After beating all 16 gyms, beat red in a battle, then Oak will give you one Kanto starter pokemon.
you have to beat Red first then go see Prof. Oak. the Pokemon are all on level 5.
You will just receive 1 of the Kanto Starter Pokémon from Professor Oak in SoulSilver meaning you will need to choose between Bulbasaur, Charmander and Squirtle.
Umm, in gold,silver,crystal,heartgold and soulsilver you can go to kanto and johto
In HeartGold and SoulSilver only Johto and Kanto are accessible. You will need to play other Pokemon games to see the other regions.