Fix Contact Form

I need a PHP contact form fixed to check to make sure user inputs the required fields. If not they get an error message (I will show example of the error message I want to display on the page) Also the form has capthca. Currently if the code entered is incorrect and user try's to send the form it take them to another page that tells them the code was wrong. And when you hit the back button the form clears itself and you must reenter all the info again. I want this to display the error message on the page they are on instead of taking them to new page. Simple fast project for PHP guy. Thanks