Every action in our system -- petitions, events, forms, fundraisers, ticketed events, and letter campaigns -- has a drag and drop form builder you can use to add extra form fields to your action. Click the blue "Add or Edit Additional Form Questions" button to open the form builder. You can learn more about the form builder here.
Questions that have not been hidden appear in the pre-built questions section in the upper right of the form builder. Questions are attached to groups, so if you don't see the questions you expect, make sure you have made your action sponsored by your group then try looking at the form builder again. Choose a question from the dropdown and then drag it into your form on the left side. Click each question to set whether the question is required or not. Click and drag each question to re-order. Click and drag and move the question out of the form section on the left to remove it from your form.
If you have the correct permissions, you can add ad-hoc questions too in the form builder, by dragging blank questions from the left and adding names, labels and whatnot by clicking on them when they're in your form on the right.
When you're done, click the green "Save Form" button to preview your work on the creation page, and save your page to save your form.