Working-holiday visas for 6 months to Japan may be granted to citzens of the countries listed below, provided that they are between the ages of 18 and 25 (up to 30 years old in some cases). This visa can be extended twice once you are in the country, for a total period of 18 months. For more information please see the Japan's Ministry of Foreign Affairs working holiday page
Native English speakers often find work teaching English in private conversation schools, and indeed in the past this was an easy to get and lucrative occupation. This however is no longer the case. Although the Japanese yen has declined in value, wages have not increased for English teachers, meaning less in the pocket to take back home with you. A typical salary would be ¥25,000 a month, which may or may not include accommodation. In addition, competition with other job applicants is fierce.
Although no official qualifications are required, doing a TESOL course (typically a 1 month full time course) before you go would both increase your chances of getting a job, and also help you know what you are actually doing. Once you have found a job, and extended your visa twice, you can stay longer if your employer will sponsor you for a work visa. In order to be eligible for a work visa you will need to have a university degree (in any field)..