Warum is an adverb = "why." Warum brauchen Sie Hilfe? = Why do you need help? Weshalb can be either an interrogative adverb meaning "why" as above, or a relative adverb or conjunction meaning "for which reason." Das ist der Grund, weshalb ich Hilfe brauche. = That's the reason I need help. Wieso can also be either of the above. Wieso brauchen Sie Hilfe? = How come you need help? Wofür can be either interrogative or relative ("for which"). Wofür brauchen Sie Hilfe = What do you need help for? I hope this explanation answers your question.