This is the first in a series of two tutorials that should help: http://acrobatusers.com/tutorials/js_list_combo_livecycle1
Here's Part 2: http://acrobatusers.com/tutorials/list_and_combo_in_lc
Ignore the stuff about LiveCycle Designer.
An AcroForm is the name given to the type of PDF form that you create in Acrobat. An XFA form is the type of form that you create in LiveCycle Designer. Although they are both PDF files, they are very different structurally and have significant;y different scripting models. So pay attention to the information on AcroForms and ignore anything about XFA. Be sure to get the sample PDF forms so you can look at how it works and where the scripts are placed.
What you want to do will require scripting, and if you don't have the experience and are unable to use the information in the tutorials to create what you want, you might consider getting someone who has experience do this for you. I'll often suggest code if I can do so quickly, but what you want requires more than a few minutes.