How can i teach my dog guarding/attack skills like a cop k9?

Ok my dog is a 10 month old pitbull, he is not aggressive at all towards anything, and in no way am i trying to make him aggressive i just need him to be an aggressive fearless guard dog and also would like him to be able to bark or attack on command like a police k9, reason being that there are crackhead neighbors. i know pitbulls are natural guard dogs but the crackheads p by so often i dont want my dog to get used to them at all