Find the website of the department of education of the state in which you wish to teach. If you already have a bachelor's degree and are pursuing additional coursework, find out exactly which courses your state requires for certification.
Research schools that offer online degrees in education. Make sure that the school you choose is regionally accredited by one of the six major accrediting bodies in the United States. If not, your degree or coursework will not be accepted by your state.
Enroll in your program and complete your classes at your chosen school. Online education requires a great deal of self-discipline; if you do not dedicate yourself to pursuing your coursework, you will not become the best teacher you can be.
Complete your student teaching experience as directed by your college. This is your chance to not only learn from your supervising teacher, but to really show what you have learned during your time as an educational student.
Once you have completed your coursework, register to take the Praxis II test or whichever state mandated teacher certification test. Once you have taken and passed this test, your score will be sent to your state's department of education and they will issue you a teaching certificate.