Disable submit button until Form is completely filled out. (Form Validation)

Hello everyone, I need to have the submit button on my website disabled until all the fields on the form are filled out, so people can not just hit the submit button without filling out the form. Basically all i want is to have the form validated before they can submit, I have attached the html file along with the php form processing script in this post, changes can be made to both files in order to get the form to work. Thank you!