How do you catch a Pidgey on Pokemon Diamond?

First get the national pokedex. Go to the house right under the Pokemon center in sandgem town. a little girl will be there. She will tell u where different Pokemon are. The Pokemon she says vary. Keep going every day. Sooner or later she will tell you that there is a lot of pidgey there on a certain route, when she says it you will hardly see anything else on that route.