Go to the Silph Co. in celadon and get the sliph scope.
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first you need to obtain the silph scope. you get that in saffron city, in the silpher co. building (its the biggest one). then when you go into the Pokemon tower, all the ghosts turn into either a gastly or a haunter. you can then battle and catch them with Pokemon.
blue, oringe, pink, and red
The ghosts can't be fought in their current forms. To beat them you have to see them and you can when you get a Silph Scope. You get the Silph Scope in Celadon City somewhere (No spoilers unless u want them)
how to get exp.share in pokemon blue
You will need to have the item called the silph scope which reveals the identity of the ghosts that ghost happens to be the deceased mother of a cubone aka Marowak you will have to defeat her in order to reach the top of Pokemon tower.