According to my knowledge you need to have a Degree in English in order to qualify for becoming a TOEFL or ESL teacher in United States. Apart from that you would also need to get a certificate in order to qualify as a teacher. If you already have your graduation degree in English, here is a link from where you can do an online distance education Teaching TOEFL preparation certificate. This is specially designed for ESL teachers and TOEFL Teachers. The course is designed by the Canadian School of Educators:
www.collegeofeducators.ca
www.collegeofeducators.ca