event on radio button

Question

Hi every body

i am developing one web application project
in my project there is one job application form
i used some labels,text box,drop down list and two radio button OK

now my question is if any user  select the radio button say yes then he will see some label display beside to radio button and if
select No then the label which is visible when he select Yes should disable how can i do this what event i have to use

please help me

thanks to all hints