This article can be a big help you understand it better.
Know the name and type of system you currently have before calling someone to fix it. This will be helpful to any contractor asks for this information.
If you struggle with your HVAC, do a tour of your house before you call in help. This will help the contractor determine where the problem is.
Know what work you need before you hire a contractor. It will be hard for an HVAC contractor to give you with an estimate over the phone without knowing what your current setup is. It will be way more hard if you over the phone without the information readily available.