A question you need to ask before you start an on-line course is how will your actual teaching be assessed? You can learn a lot about teaching but it's different when you put it into pracitce. Check with the course provider how you get feedback (if you get any at all).
It might be a good idea to get a certificate before you start on the diploma. Or this might jus tbe the name that's given to it. Diploma is usually a couple of years after the cert.
hope this helps