Hi Truong Khanh,
You need to contact the people who run the Online Program where you enroll for the Self-Study class. They can answer your questions. E-mail them directly and ask them about the class you signed-up for. You are asking specific questions they alone would have the answers for you. Good luck!
Two things may have happened already. First, the certificate is enroute already to your home address. Second, the class you took doesn't provide a certificate. Most online schools would have students PRINT their own certificate upon course completion. Check your inbox to see if the school has sent you the certificate already. You can also write the teacher who taught your class---by hand. All course information---as well as contact numbers should be listed in your course syllabus. I hope you can resolve this problem soon.
======MerriLee LeonardM.A.,Ed.,TESLNorthern Virginia Community CollegeU.S.A.=======