How do i make my dog more protective? Like bark when someone comes over or if someone breaks in?

I know that I can't make her bark but I love her to death she is so cute. She only barks when your playing with her.How do i make my dog more protective? Like bark when someone comes over or if someone breaks in?
Proabably take to a trainer, unless you yourself are a trainer, because dogs (most of them) can be taught to warn.