How do you get all starters in Pokemon HeartGold and SoulSilver?

Answer 1

If you mean the Johto starters (Totodile, Chikorita and Cyndaquil) then you can get one from the Professor at the start of the game, the one your rival has via the GTS (because you've seen it), and the last by wireless or wi-fi trade with a friend (I think).

For the Kanto starters, get all 16 gym badges and then defeat Red. Then you can go to Professor Oak in Kanto and he will offer you one of the Kanto starters. DON'T PICK ONE IMMEDIATELY! Look at all of them first to get the data in the pokedex and then choose one. The other two you can get off the GTS because you've seen them both now.

For the Hoenn starters, defeat Red and then go talk to Steven Stone. He is in Silph Co in Saffron City. He will offer you one of the Hoenn starters and, as before, DON'T PICK ONE IMMEDIATELY! Look at them all first to get the pokedex data and then choose one. The other two can be traded on the GTS because you've seen them now.

Also, a good idea is to breed one of your starters with a Ditto, hatch the egg, and then use the hatched Pokemon in the GTS. Offer that starter for one of the other starters you've seen but don't have. This is a really good idea as I got Bulbasaur, Charmander and Totodile all in a couple of hours with this method.

Hope that helped!