How do I make a checkbox default selected on a form?

I have an option at the bottom of my page where i want the readers to have to deselect the box if they do not want to receive our newsletter. 

How do i make the box already pre-selected on the form i made?

You may have the checkbox selected by default by clicking on Selected icon. Please see the screenshot below:

You may also select this option from the Advanced section of the field properties.

Hope this information helps!