How can i get installer.App on My iphone 3GS ?

I want to get installer.App in my iphone 3GS but i don’t know how , please i need some help ! and was i supposed to have in already in my iphone when i bought it ?? and how can i get it ?? i really need it to download cydia from it , and if i can’t download cydia from it , i’ve heard that i can download “Installer.App” from “Cydia” It’s self is it true ?? and how can i download both of them !! help help please !!