How to input pop up calendar?

You have to use our DateTime field under Quick Tools section in the form builder. Or, you can any of the following date widgets:

http://widgets.jotform.com/widget/date_picker

http://widgets.jotform.com/widget/mini_date_picker

Here's how to add a widget in your form: http://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form.