ASHRAE has a set of handbooks which are pretty good, but to be honest, they don't really tell you how to be an HVAC engineer. In most cities, there is an ASHRAE course that will give you a lot of the basics, but it is generally targeted at non-college grads. The basic things you need to know to be useful right away are CAD and how to follow directions. Other than that, being able to think like an engineer will help you learn how to be an engineer. That's what college is really for.
My first day at my current job, I was handed a slide-rule type device called a Ductulator, given a couple of rules of thumb, and told to start sizing ductwork. Basic HVAC design is just:
Computerized load calcs->Select a system/unit->size and draw ductwork
I'm lucky to have a boss that likes hands-on work, because a lot of young engineers don't do much more than that for several years. Either way, though, there is an awful lot of learning by experience in this job.
One way you might impress (or fail to impress) a potential boss is by your understanding of psychrometrics. There are some counterintuitive things about humidity that even a lot of experienced engineers get wrong (ie, too big of an a/c unit can make a room more humid instead of less).