Automatically populating text fields

Hello,
A most awesome piece of software, and a terrific value.
I have my add new form that the user will select a value in a drop down list populated by a table lookup. How do I get the associated fields (designated as hidden fields in the format screen) to populate text fields on the same form.
I guess this is a dynamic drop down question, and I saw a post from earlier regarding this, but it is still just not really clear to me. Seems to me I would designate the fields as hidden , with my 'lookup' field, then 'assign' that value somehow to the text box on my form....or something....Any suggestions would be greatly appreciated!
Thanks. <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=442&image=1&table=forumtopics' class='bbc_emoticon' alt=':(' />