New To Javascript - Help!n

Im new to javascript and have no idea what I am doing. Im making a form which, when a check box is marked, requires another field to be filled. In my screenshot, there is a check box if a person was referred by a doctor. I want to require the name and phone number of the doctor if the check box is checked. I have been looking around the forums but I don't seem to be able to find an answer to my question. Im certain someone knows how to help me with all of this great knowledge in the forum. I appreciate your help. Please keep in mind, javascript is NOT a forte.