Both Chrome and Firefox are trying to "encourage" everyone to use SSL, so they issue that warning on any page that without an https URL that asks for user information.
I've been happy with this free SSL certificate, which will work on a shared server:
LetsEncrypt. Your host may install it for you if you ask. Some better hosts provide it as an option in cPanel. You would still need to modify things to make that page be automatically served with an https URL. Or you could make the whole site be served as https if you have other forms on the site, e.g., a contact page. This used to slow sites down, but it doesn't any longer.
As others have suggested, what you have now it not all that insecure. The SSL certificate mainly ensures that the server serving the page is the server it claims to be. As long as your site hasn't been hacked, you're fine.
