First of all, you need to do as much research as possible. Read online travel guides such as Wikivoyage and have a good look through the Lonely Planet travel forum for relevant articles on any countries you are especially interested in. Search for personal blogs written by teachers who are already living and working in the countries you are interested in. The bottom line is the more research you do before making such an important decision, the less likely it is that you are going to be shocked in a negative way once you have arrived.
The first decision is probably to decide on a particular region. There are four main regions of the world where you can teach English as a foreign language. They are Asia, Europe, the Middle East and Latin America. Each region has its own advantages and disadvantages, and furthermore so too does each of the nations within each region.
Climate is a very important factor to consider. How well do you cope in high temperatures? And what about chilly weather and snow? If you have a particular dislike of hot weather then it is probably best to avoid countries which have high average temperatures every single month of the year.
What are your hobbies? If you like diving, consider countries where the diving spots are widely considered to be world-class. It’s not all about work after all. How happy you are outside of work is crucial to your overall enjoyment of your time overseas. If you can pursue some of your hobbies in your spare time then it is a definite advantage.
What is your position with regard to alcohol? If you are a regular drinker and keen socialiser, it is probably wise to avoid the majority of countries in the Middle East in which alcohol is very difficult to come by and public drunkenness a serious crime.
What is your nationality? Some countries make it easier for teachers of certain nationalities to get the all-important Work Visas. Check what the latest requirements are on the relevant Embassy websites.
What is your educational background and age? Different nations have different requirements of their English teachers such as having a university degree, or being over the age of 25.
How much money do you want to make? If a high salary is important to you, look at the countries where you can save a significant portion of your monthly wage.
What are the hiring seasons in your country of choice? Especially outside of Asia, you really need to make sure you get the timing right or else you might end up waiting several months to be hired.
Do you want to be hired from overseas or would you prefer to arrive in the country first and look for opportunities once you are already living there?
How do you feel about living a long way from home? If you prefer to be within a three-hour flight of your home country then get out a map of the world and look at the options you have narrowed down. Obviously, if this is not an issue for you then you have a lot more possibilities and opportunities open to you.
Remember that wherever you are for your first year of teaching English abroad, it doesn’t mean that you have to stay in that country, or even that region, for your second year. There are so many opportunities out there that it is common for a TEFL teacher to live in Asia one year, and move to Europe or Latin America for his or her second contract.