Obtain a bachelor's degree and a TESOL (Teaching English to Speakers of Other Languages) certificate. While requirements may vary, a bachelor's degree in any subject is acceptable. In some cases, you will need a master's degree in your chosen discipline as well. Teaching experience is preferred, but not always necessary.
Learn Spanish. It will be much more difficult to teach Spanish speakers your language if you cannot speak theirs. Make sure your own English skills are up to par.
Contact a school in Mexico City and ask if they are hiring new English teachers. Websites like solutionsabroad.com can help you locate potential schools. Note that while cold calling may help you to acquire a better position, there is no guarantee of placement. Many of the top schools will only consider applicants with a master's degree and years of teaching experience.
Go through a placement agency. Organizations such as the International Teacher Training Organization (ITTO) or the Language Corps can provide on-site training, certification and a job. This will come at a cost to you and you may not be placed in Mexico City. However, previous teaching experience in Mexico will place you in good stead if you should choose to apply later on.
Go to Mexico City and look for a job once there. It is possible to find a greater variety of opportunities this way, as Mexican schools do not often advertise for English teachers on job websites. In addition, you will have a face-to-face interview, which can give you an advantage.
Apply at Berlitz or another foreign language school. Foreign language schools typically cater to adults and pay by the hour. You can also try conducting private classes on your own. Ask an established private English teacher if he has any extra students you can take on. Advertise your services in schools, local businesses and through word-of-mouth.