Hi Colleen, all you need to do is edit the form. Or clone it, and create a new form with the new question (field) included.
Instructions on how to add new fields to your forms can be found at http://community.marketo.com/MarketoDeepDive?id=kA5500000008QNcCAM
Just to clarify Adam's comments. If you want to change a field label, you just need to edit the form. If you want to add a field to your form ant that field that doesn't exist at all in your Marketo account, then go to the link Adam provided in order to create a new custom field in Marketo. After it's created, you can edit the form and drag it in.
However, I assume that your use case is the most common one, which is that you have an existing field in Marketo that you want to add to an existing form. For this use case, you just need to go to the form editor and drag the field into your form.