Go to "Pages" -> "Signup form" and click "add field". You can choose what sort of field it is under "type".
The different types.
"Short answer", is quite self-explanatory — a short written answer.
"Paragraph" resembles "Short answer" in that you write your answer, but this type allows for it to be longer.
"Multiple choice" lets you pick one of several visible option.
"Checkbox" is also a multiple-choice answer, but here you can select more than one alternatives.
"Dropdown list" is just a different kind of multiple-choice list where the answer you choose remains the only one visible.
"Country" is a dropdown list with countries as a preselected answer.
"Title" is a headline, which can be used to specify what kind of question it is.
"Description" can either be used to pose the question or, if you used the title for that, you could elaborate on it.
Under description, there's a box named "Required" you can check to make the question mandatory to answer.