How to create custom sign up form which would update user profile fields.

Hi. I really need help of somebody as I completely don't know how to do something I need:
The task is: I need to split sign up form into two pages so that users enter part of their personal information on first page, then they enter some extra information on the next page and then submit the form.

I suppose to build a custom template that would show first and second page, but I couldn't find a way of how to insert form fields (the same form fields that are used on generic sign up page) into it.
If I could call form fields somehow in PHP to print them in my template....
Does anybody know how to do that?

Probably, someone knows how to change PHP/HTML code for sign up form ( I couldn't find which file(s) is responsible for HTML generation for sign up form).