How can I format text in a text box

You can accomplish that by using the Designer Feature, simply once you are on the edit mode of you form:

1) Click on Designer:

2) Click on CSS tab, then click on the text box you want, and click on "Font Family", there enter the font family type you want, in my case I entered Comic Sans MS:

3) Click Save, and then on Preview, you will see the changes:

You might want to see our Form Designer tutorial to help you create fantastic forms!: http://www.jotform.com/help/300-Form-Designer-Tutorial-Let-s-create-fantastic-forms-