I began my TEFL career forty-three years ago. I am still learning about the language and the ways in which it is used. Even if you 'knew' everything before you started, you would soon discover that you didn't!
If you have been accepted on a CELTA course, then the course providers believe that you are capable of completing your course successfully. You will learn a lot - and un-learn a lot. The most important thing you should learn is that you do not stop learning when you complete the course.
Good luck - and don't worry too much.