First of all, we think of teachers as professors - highly educated academic types, even teaching at an elementary school requires a degree in teaching, often a master's degree or PHd.  Teaching English should at least require a degree in English - and it does.  I often hear people saying they are a "Sensei", which makes me laugh for a few reasons.  First of all, the term "sensei" means something like "Professor" in English, and is usually reserved for those with teaching degrees or some sort of doctoral degree. (In fact, medical doctors are also called sensei in Japan).   Most of the people using this term on themselves aren't any sort of tenured professor, but someone who just barely met the immigration requirements (having some sort of undergraduate degree from a college).  Secondly, "Sensei" is a term of respect, meaning you shouldn't use it on yourself.  That would be like a Judge in the U.S. calling himself "My Honor".

Another problem I have with hiring anyone off the street and assuming they can teach English is that... well just because English is someone's native language doesn't mean they are good at it.  Many people in the US and UK are horrible at English.  Poor grammar, Lazy speech, and local slang abound.  Many people who become English instructors in the programs below have graduated in completely unrelated majors, and had barely passing grades in English - the very subject they are supposed to teach!  In fact, having a degree at all is only a requirement of the Japanese government for granting a work visa, not something the schools actually care about.   (Actually, for those eligible for a working holiday visa, even the undergraduate degree is not required).  Still, many of the companies below further the problem by actively recruiting with slogans like "No experience needed", etc.  They say it because it's true, but that should be a warning.  Think about it, what if a law firm wanted to hire you as an attorney and put up a sign "No experience or knowledge in law needed!"  Would you want to work there?  Would you trust that law firm to handle your case?

Technicalities aside, there are basically two ways for an unqualified person to teach English in Japan:

1. Join the JET Program -  This is a program sponsored by the Japanese government, which encourages foreign graduates to come and become assistant teachers in Japan.  Although you will only be an "assistant" teacher (meaning you will help people with pronunciation, etc.), you will help out in a real high-school.  JET is meant for fresh graduates of undergraduate schools, and so one can hardly expect that most of them will speak Japanese.  If you plan to come to Japan on JET, though, it would be a huge advantage to you if you made your major English, and your minor Japanese (or vice versa).  Many Japanofiles overseas have visions of Shibuya and Akihabara dancing in their heads, only to be shipped out to the country-side when they arrive in Japan on the JET program.  The reason for this is simple, the cities are more than saturated, and the country-side is, well, lacking in foreign teaching assistants.  I am not saying that living in the country-side is a bad thing, I am just saying: brace yourself.  City-side Japan is fairly modern.  The country-side, depending where you are, not so much.  People who arrive on the JET program can only work a limited time, and so upon completion of the program, either have to go back home, or find another job that will sponsor their visa before then.  JET may not be wonderful, but it is a good way to get an introduction to Japan, and you know what you will be getting yourself into to some extent.

2. Join one of the large private English teaching (eikaiwa) companies.  Nova, Aeon, ECC, Berlitz, etc.  Note that these are not accredited educational institutions, but just companies who offer classes.  As such, their instructional quality is not regulated in any way, and the result is what you would expect.  In the quality continuum, Nova is one of the lowest, and Berlitz, with it's business focus, is one of the highest.  Because they want students to sign up and pay more term after term, they of course typically pass students (assuming they have grades at all).  Most of the students at these schools are going more out of entertainment value than for actual rigorous learning experiences.  After all, they would take courses at an actual university if they wanted quality education.  Nova went bankrupt in 2009 after several months of non-payment of employees' salaries, leaving many thousands of former instructors jobless, and in many cases, homeless.

The companies mentioned above make their money off of mass-produced edutainment, and so volume is key.  As a result, they go for quantity over quality, and thus want to recruit as many instructors as possible - that's why qualifications are often very lax.  In the end, though, one always gets what they pay for.  This applies to the students as well as the teachers.  

This means two things.  First of all, of you are an actual qualified teacher, then there is no way you would want to work for most of the companies in this field.  Second, if you are someone who wants an easy way into Japan, then this may be the path for you, but since the only requirement is some sort of undergraduate degree, expected to be treated like the replaceable livestock you are.  Think of how many other people also wouldn't mind coming to live in Japan for a while and want to take the path of least resistance.  This isn't to disparage the skills or talents of those who work in the English schools, but just to warn them that they might not be able to put such talents to use. 

One of the advantages of a place like Nova vs. the JET program, is that a prospective employee will potentially have more leeway over where they want to work and live.  I say potentially because the same supply and demand forces are in play in the private sector as in the actual schools.  Still, the chance of being able to land in your desired location is higher if you apply to one of the companies.  Yet there are consequences to this as well.

Salary is a bit of a misnomer, since the schools typically pay by the hour or lesson.  The first thing to know is that since the English edutainment companies basically hire anyone with a pulse, is that since you are replaceable, you will be paid only the minimum legal wage.  In Tokyo, that amounts to about $2500 USD per month minus taxes, or about  $2200 after taxes.  If you live in a normal (but very cheap) apartment, you are looking at around $600 per month or more.  You can also expect to pay about $50 per month for electricity, the same for gas and water and internet.  Cell phone bills can run about $100 per month or more depending upon your usage.  Add all that together and we've got $900 a month for basic living expenses... plus the train fees (maybe an extra $100) if your company doesn't pay.  Usually you will want health insurance too, and who pays that varies by company.  So then you take $2200 minus $900 and arrive at $1300 to live on for the month (optimistically).  That's about $43 per day.  When you realize that it's very easy to spend that much on food alone, much less things like drinking with co-workers, you will realize that this level of salary is just barely scraping by.  Plus, even if you do find an apartment for $600 per month, it won't be very nice.  Your best bet is a "guest house", which is somewhat like a dorm.  Another thing you need to know is that it's difficult for foreigners (especially those who don't speak Japanese!) to find housing in Japan.  If you do find a place that will accept you, you will still need several months rent as a (typically non-refundable) move-in fee, and someone to sign as your guarantor.  Also note that most of the English schools consider their instructors as contractors, not employees, meaning you will receive no benefits.

For a more realistic example, Gaba, as of 2010 hires with a starting pay of 1500 yen per lesson (which may, or may not increase slightly over time).  If you somehow manage to teach 40 lessons per week (which would be truly amazing), that would come out to $600 a week, or $3600 a month - if you work weekends and holidays.  That sounds not too bad if you are from Kansas or another low cost area, but consider that the $3600 is before you pay taxes, Then you're looking at about $2800, before health insurance, and assuming you can keep up 40 lessons a week without any sick days or breaks.  Upon consideration, you can see that the $2500 figure listed above is actually generous.

On the other hand, many of the schools offer housing!  This is indeed convenient for those just starting out in Japan, as it eliminates a major hassle for you.  On the other hand, you are actually just trading the hassle of finding a place for the hassle of living at the type of place that the English schools offer.  They often pack people very tightly into low quality housing.  The prices are actually cheap, but a usually poor value for the size and location.

Then there's the educational process itself.  Factories like to keep their equipment utilized as much as possible at all times in order to maximize profit, and since the English schools are essentially factories, they also like to keep their equipment (you) as busy as possible.  This means few breaks and fast-paced lessons, often with little deviation from the script.  The instructors are expected to be Educator, Entertainer, and usually salesperson.   As an example, when Nova was still around, they offered two minutes before and after each class for lesson preparation and grading.  There is a reason that 96-97% of all instructors at such schools stay in Japan for less than three years!  ()

None of this is meant to say that if you work at one of these places you are a bad person or such, simply that many people get tricked and trapped into working for this type of job.  If your goal is to come to Japan at any cost as soon as possible, this may very well be the easiest way - this page is simply meant to provide ample warning.  In the end, being an English instructor is not typically a fun experience for most, and thus is usually the path taken by those with no other options as a last resort, or those who are deceived into it.

If you want to be happy in Japan longer term and are willing to put in the time, I would strongly suggest learning Japanese and developing some professional experience working overseas first.  Then you can either try to transfer to Japan (if your company has an office in Japan), or attend Japanese career fairs until you get an offer.  Japanese companies do hire foreign employees in various areas of the economy, including Consulting, Finance, IT, etc.  If English teaching is truly your thing, then you will be happy to know that Japanese Universities also hire qualified teachers.  (And the level of salary and respect you receive will of course be higher than you can garner by either JET or the private English mills).