Eugene Potterton

OK I am new to this and have had some success with using html, however I seriously need assistance in creating a certain part of the form that is already created. I have insert 2 radio edit buttons, the first being YES and the second being NO. How do I create or right the code so that when no is selected a new required text box appears. Sorry for the unintelligent question but I am at my wits end. Regards Eugene