First decide where you want to work. It's a big wide world, so even narrowing the choice down to a continent would help. Then spend at least a couple of weeks studying the Internet forums and boards.
Read first. There are few things more irritating to board members than new members who obviously haven't read any of the existing posts jumping in and asking the same old newbie questions. Most if not all boards have a search function. Make use of it or you'll get a lot of sarcastic and nasty replies pointing you to it anyway without answering your question. Try to be as specific as possible, the more information you give in a question the more likely it is someone will be able to give you a sensible answer.
Start here of course, and look at Dave's ESL Cafe http://www.eslcafe.com/
which is equally wonderful and has some of the best job forums around. My tips are:
1. Never send anyone any money to secure a job. Anyone asking you to send them money is scamming you. The whole point is that they pay you
to teach English. Reputable schools make their money from the learners, not the teachers. Also never ever
send original documents like passport, degree certificate or teaching certificates. You may have to send them to get your working visa once you have a job offer, but make sure it is a cast iron guaranteed offer of a legitimate job with a legitimate company before you even consider it. One of the most common scams is to have people send their passports etc. which are then sold on the black market and you never see them again.
2. Avoid agencies wherever possible, unless you know without any doubt that they are above board and can get you a job you would not otherwise be able to get without them. Agencies are all well and good in certain circumstances, but it is much, much better to contact schools directly wherever possible. You will be saving the school money this way, which may help you to get the job you want.
3. Wherever possible, talk to or email people who have worked for the same school or organisation directly. Beware of 'testimonials' on websites. They may have been written by the school itself, and the person giving the testimonial might not exist or have any idea their name and photograph is being used.
4.Find out as much as you can beforehand about the culture and language of the country you are going to. Not only will it help you enormously when you get there, but it also shows you have some degree of respect for the country and it's people, and are not just another foreigner turning up to take advantage of the cheap food and beer and the hot weather. If you can't learn some of the language before you go, take a phrasebook and an English/relevant language dictionary with you.
5. Be wary of what you read on the EFL forums about people's experiences of a countries culture and customs. Sadly, a lot of the people who populate the forums are the kind who just want to vent about how nothing in the country they ended up in is as good as it is back home. You will hear a lot of complaints in the China forums for instance about how corrupt it is, and how nothing gets fixed or dealt with on time. Well, it's China, what did they expect? Sadly some people think the whole world should revolve around them and respect them wherever they go. Having to deal with a different pace and way of life is part of the fun of travelling is it not?
6. Make sure you take enough money with you to get by in your new country for at least the first month or until you get paid. Your employer should be able to let you know what the cost of living is locally. When you arrive, avoid the habit of working out how cheap everything is compared to back home. A beer may cost half what it would back home, but you are probably getting paid half what you would once you converted the currency. Adopt the habit of working out how much things cost relative to how much your monthly salary is and you will avoid running out of money. Also if possible, make sure you have enough money to get a plane ticket home if you find in the first week or two that it wasn't what you expected and you simply have to go home.
7. Check with your new employer what clothes you will need before you pack. No point in taking winter clothes to Thailand. Take as much 'realia' as you can, photos of your home and family, a small flag of your country perhaps. The students you teach will be as keen to learn about your life back home as they will to learn English, and it's all useful stuff in the classroom. Take as much music as you can for teaching too, the Beatles are good, their English is clear, nursery songs and the like are good teaching aids as well. Pack a few ,desert island' food items too. A bottle of your favourite sauce, tea bags, anything non perishable that you don't think you'll be able to find when you arrive will help stave off the homesick blues if used sparingly.
8. Lastly, get a Skype account set up if you don't already have one. It'll save you a fortune in the long run if you have Internet access, and if you pay some credit into it before you go you can use it to phone banks and other institutions you may have to get in touch with at local rates.
Sorry for the long post, but I think I've covered most of it. Above all, be respectful of your new country and it's people, and enjoy it as much as you can.