Decide how much time you want to devote to taking classes online. Most online classes offer 60 hours, 100 hours, or 120 hours of online class time. The more hours spent, the more material is obviously covered.
Research the curriculum of the institutes that appeal to you. Online degrees are available from companies including teflcorp.com, teflcertificatecourses.com, englishjobmaze.com and worldwidelearn.com, among others. Make sure that what they offer is something you would want to study or that you think will be useful for you later down the road.
Ask about the institutions' own certifications. Make sure they are certified for online teaching and for offering classes in their own state. Research the teaching laws and requirements for offering classes online to make sure they are creditable.
Talk with someone from that institution. Make sure all your questions are answered, if you have any, and when the time comes for payment you are sure your money is going to be received by your chosen institution.