How to specify authorize.net on a donation form

Hi:  new user!  just setting up my first form...it will be for donations to our non-profit organization.  I picked a donation form template and when i went into settings it only shows PayPal.  How do i get it to use authorize.net as an option?  thanks

You  need to first remove the current payment integration in your form, here's an image for visual guidance, you need to click on the field to get the options: 

Once deleted you only need to go to the add form element button, payments tab and add the Authorize.net integration from this list: 

If you have any question, let us know.