Successfully completing a course will automatically reward you with a certificate.
For some courses there will be questions for you to answer in order to earn a certificate; the pass rate required for these tends to be 80%. If you do not achieve the required score, you'll be able to attempt the course again. You could choose to do so straight away or at another time.
If a course doesn't have a test at the end, please do not worry. As long as you have watched all of the content of the course it will have registered that you have completed it.
Once you have earned a certificate you will be able to download it as a PDF or alternatively you can access it from the course page when needed.