The only required field in your signup form is the email field. You can add as many as you would like, and decide whether or not they are required. If you're not sure what data you need to collect, Name and Email is usually a great place to start. You can always expand your form later.
Custom fields include text boxes, radio buttons, checkbox lists, and date entries.
The Email field is always required.
Your field changes are saved automatically, as you make them.