To your volunteer application, add the physician contact fields that you want the volunteer to complete. These should usually be added as user fields, though you could use custom fields as well (using custom fields means that you can only effectively allow a volunteer to apply once: the second time they apply it will overwrite the fields from the first application.)
At a minimum you must capture the physician's e-mail address - that's the address that the notification to approve or reject this application will be sent. Right-click on that field and select Validation --> E-mail address. In addition you should capture the physician's name and possibly verify the e-mail address (using the Requires a specific value right-click option).