Plan on living in Japan for the long term? Then you will need a visa!
Coming and going out of Japan is relatively easy as long as you are in good standing with the government. You can basically show up, get a 90-day tourist visa and enjoy your time here. However if you wish to stay longer then that you will be required to obtain a visa that gives you that capability.
There are 3 basic ways of obtaining a visa (there are more than that, but all can be related to the 3); first is a working visa, second is a student visa, and third is a business visa.
To obtain a working visa you must either have a Japanese company sponsor you or be a transfer from your countries corporation that has offices in Japan. If you plan on coming to Japan and just start a new job it is possible if your country has a working holiday agreement, if not you must first be hired by the company while outside of Japan and then they will set up all the necessary paper work with their lawyers and accountants. There is a little hoop jumping to be done, but all in all if the company is legit then you will have no problem acquiring the visa.
The second is by obtaining a student visa. With this route you must have finished high school and pass some other requirements, such as around $20,000 in your bank account. The first step in this process is picking a school (doing a web search is probably the best way). There are many Japanese language schools throughout Japan, however the cost of living is high pretty much in all big cities, and unless you are independently wealthy, you will need to work which is allowed part-time while you have a student visa. Thus I suggest you stick to Tokyo or Osaka where work is abundant.
The third and by far the most complex is obtaining your own business visa. Here you will need to put a minimum of 5,000,000 yen in a Japanese bank account or partner with someone (requires less start-up money). Depending on what kind of business you want to pursue I would suggest taking on a partner, unless your Japanese is flawless. Having a Japanese partner doesn’t mean they will be involved with all aspects of your business, but it’s nice to have a local in your corner should some problems arise. There are many other rules and requirements such as hiring local Japanese people and because of that I suggest also hiring a lawyer and talking to an office which handles visa paper work.
Visa offices can answer questions via email and phone calls but ultimately you will need to speak to them in person and pay consultation fees. Some of the services available are investor visas, resident visas, company formation, tax return filings, business licenses, obtaining a office or retail establishment, working visas, and Japanese citizenship.
Good luck and enjoy your time in Japan!
More info on visas in English can be found here:http://www.us.emb-japan.go.jp/english/html/index.html