You can't, Bulbasaur is unobtainable. You have to trade, hack, cheat or get it from mystery gift.
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You cannot catch bulbasaur in diamond and pearl. If you have firered or leafgreen can set 6 Pokemon free in pal park and catch them there.
Bulbasaur is a starter for the Johto Region, so you cant catch him in the Sinnoh.
get firered or leafgreen and migrate it to pearl after national pokedex
you do not catch it . you can bulbasaur when you get it from proff oak
you can not catch bulbasaur you have to pick him as your starter Pokemon