Teaching English and living in Japan is a great experience, the adventure of my life. I've been doing it for over 10 years. I would encourage anyone to come here to start a new career or just for a couple of years for the experience.
I know many people do the latter. If you are really adventurous then go for it. But I recommend the former. There are many Eikaiwa, or English Conversation schools in Japan. They're private companies where Japanese come to study English. The average starting salary is between 250, 000 to 270, 000 yen, about 2700 to 2900 dollars per month. I applied to AEON Corporation while still in the states. I interviewed in Chicago and later came here. I no longer work for Aeon, but I recommend them. I was with them for almost 9 years. That's probably some kind of record. Most teachers are there for 1 or 2 years. They are probably the best of the big conversation schools left. A few years ago, the number 1 school, NOVA went bankrupt after a huge lawsuit. They had a bad reputation for years with teachers but also students. It was finally a class action suit from former students that broke them. That event really hurt the industry, but Aeon is still a safe bet. But be warned, they expect a lot. They have high standards of professionalism and they really drill that into you during training. You have to wear a suit and tie. They have branches nation wide, some schools have only adult students, many have adults and children. There are other schools as well, GEOS and Berlitz to name a few. Be wary of one called G. Communications. They are the ones that took over Nova. I wouldn't trust them.
You can also become an ALT (Assistant Language Teacher). ALTs work in junior or senior high schools. The JET program run by the Japanese government is probably the biggest source of ALTs 神奈川区 塾. Interac. Co. is one of many private companies that provide ALTs. I think Japan is ready for a mini-boom for teaching English to children. Starting in 2010, English will become a mandatory subject in elementary schools nationwide, so i think there's a lot of opportunity there.
The best place to start is at Dave's ESL Café. It has an abundance of information, not only on jobs, but teaching resources and ideas. It's huge. Be sure to check out the teacher's forum. Read the comments to see what teachers say about different companies, avoid the bad ones. For jobs, Gaijinpot and Ohayo sensei are good. All about teaching English in Japan is good too.
How about education? Most big schools require a college degree. It's not necessary to have a teacher's degree. It's probably possible to get a job with only a High school diploma, but you will be rather limited. Another thing to consider is getting a TEFL (Teaching English as a Foreign Language) certificate. These are relatively easy to get, only taking a few months. They are available online and most offer an additional practical teaching workshop at a specified location. I recommend that if you can. Most English schools don't require such a TESL certificate, but it can give you a big advantage. It looks great on a resume.
If you are living in Japan now, then there are lots of options. It just depends on where you're willing to go. Actually, in my case, the most important thing in finding a new job was word of mouth. I have a family here and I didn't want to move to a new city. Jobs were limited and I never would have found my current job if it wasn't for my friends. I'm really lucky. Never underestimate the power of networking and friends.