As a former English teacher in Japan I had some extremely good times as well as some not so good times. Japan can be a vibrant and lively place full of friendly faces and new experiences, though you must remember that there is of course ying to every yang.
The Life of an English Teacher in Japan

My intention is not to put you off, seriously going to Japan as an English teacher was one of the best things I ever did, however there are certain things that you need to be prepared for in the working world as well as living alone in a foreign country.
Applying for a Job as an English teacher in Japan may seem daunting at the beginning. Most English conversation schools do not require any specific English qualifications, the only requirement is to have a degree … in anything.
Once you have applied at a few companies you should be prepared to undertake a skype interview. You should always dress smart for these interviews much like you would for a real interview. Speak clearly, slowly, respectfully and be enthusiastic, they will be looking for all of the qualities of a great communicator. Most of the time you also don’t need to know much Japanese.
If and when you are successful you could be asked to hop on a plane almost the next week, so make sure you are prepared and have the funds.
You should aim to take around $3000 with you, $1000 to get you through your first month and $2000 dollars for emergencies. Trust me, you cant be to careful, ensure you bring as much as you can in case of emergency situations. Once you are settled and start working you should be able to save, no longer relying on finds from home.
When it comes to accommodation there are certain things you need to consider. Firstly most companies will source accommodation for you, I think this is the best way because its hassle free but you can of course take the challenge of finding your own place.
Secondly you should realise you are going to be sleeping on the floor, most lower budget accommodations will be Japanese style and will sleep you on the floor. It does get better after a while though.
Thirdly you should familiarise yourself with the area slowly, don’t just go out and explore as you will easily get lost.
When it comes to actually teaching English you will be required to wear a shirt and tie each day (ladies the equivalent) with a smart hair style. You will undergo some initial training but not much as they will want to get you making them money straight away. You wont need to know much when it comes to the training though, most lessons just follow guided text books.
If you can, you should try and befriend the staff of the school and the other teachers as soon as possible to broaden your social network. Things are always a lot better with friends and people you can talk to / do things with. Becoming friends with students is usually off limits.
In many people opinion people say that teaching 50 minute lessons 7 times a day can be mind numbing. I do agree, however I believe approaching lessons with enthusiasm and knowing you are helping people will make a big difference to your daily outlook
Communication with loved ones, family and friends back home is really important for your mental stability. Try and communicate and chat through skype regularly to keep yourself social and get things off your chest, worries, anxieties etc.
When you are faced with problems in daily life it is hard to go home by yourself and have no one to talk to, this is where things can get dark sometimes. That’s why its important to build a good network of people you can rely on in Japan as soon as possible.
Ensure you take time to schedule things too, like day trips, weekend getaways etc, make the most of the opportunities you have in that wonderful country. However usually you are very luck if you get 2 consecutive days off as English teacher, they are usually spread apart.
The most important thing though, learn Japanese. You will never learn Japanese in a better situation. You are living it! Keep striving forward with Japanese, you will improve quickly and life will get easier and more enjoyable.