How do I fill a field's value based on the value of another field?

How do I fill a field's value based on the value of another field?

When I am in the Cases (or other modules) of CRM and Choose an existing Contact (lookup), the associated account is automatically filled in for the Case. I would like to do the same thing on Creator form's I have created. The first thing I would like is exactly what I described for CRM Cases. If I get that to work I would like to fill other fields (e.g. phone number based on contact).

I am new to Creator programming. I've been searching the forums, KBs and help for this problem. I've found a few pages but nothing that really describes the specific steps I need to do. Depending on where I edit the form, the pages often look entirely different from the help page examples.

I would appreciate some detailed steps I could take that includes where best to do the edits, should I add the code to "On user input" or elsewhere, etc. Programming per se is not a problem for me - I've programmed in PHP, Python, Perl, basic, fortran etc. But I'm not finding the help I need and would appreciate some assistance getting started.