Pass Data From One Form To Another?

I have been searching everywhere for the proper method to pass input from one html form to another. I have found a bunch of javascript that is supposed to break down a query string and populate the second form based on that, but no matter what i try i cant get it working. Basically, im trying to have a small form on the main page where a user would enter some basic info (name, etc.) and also choose from a drop down menu for one selection. Then on clicking submit, that would launch the full form, populated with the data they filled out, and including new input fields.