This is from AP. It should be noted that the rules can be a bit wishy washy - for example the hyphen is "optional in most cases, a matter of taste, judgment and style sense." Be warned, however, that there are still a lot of conservative editors out there who believe just the opposite and go overboard on the use of hyphens in compound modifiers.
Please read this through thoroughly and absorb it.
hyphen (-) Hyphens are joiners. Use them to avoid ambiguity or to form a single idea from two or more words.