Load model with Ajax and jQuery to load a form

Hi all, I am having troubles to figure out how I can load data via
ajax and inject those data in a form.

Basically, I have a table of users. Each user has an edit button. On
click, I want to load this user's data and pop out an modal box
containing a form filled with the users data so I can edit them.

Creating such a thing to create a new user was fine but I don't see
how I can populate the edit form.