web-to-case is used to create web form or a self-service customer community to make it easy for customers to submit cases directly to your customer support group.
to set up auto response rule for web-to-case,you will have to follow the steps:
in your quick search box..type web-to-case.
click on web-to-case and you will get basic settings and auto-response email setting.
enable web-to-case checbox and choose case origin like:web.
choose email template and write your email signature.
click on save button to save auto-responsive email.