Flying From the UK
Direct flights only go from London, are either operated by British Airways or Japan Airlines, and take around 12 hours. These options tend to be more expensive, and if you're willing to take a layover, usually adding about three hours to the journey time prices drop significantly, with departures from most major British airports.
From the USA
Direct flights go from most major cities, including LA, New York, Washington, Denver, San Francisco, Boston Dallas and Atlanta. Prices and airlines vary by city, with New York and LA being the cheapest, and ANA, Air China and Air Canada among the most common airlines.
From Australia and New Zealand
Direct flights go from Melbourne, Sydney and Auckland with Air New Zealand, Cathay Pacific and Qantas the most common operators.
Tokyo has two international airports: Narita and Haneda. The majority of domestic flights from Europe land in Narita with flights from the States and Oceania varying between the two. Haneda has more routes connecting various Asian countries, but if you have a stop over in Hong Kong for example, it's relatively likely you'll land here.
Tokyo Narita Airport
If you're planning to visit other cities in Japan before Tokyo it's common to get to Tokyo by train.
From Osaka and Kyoto bullet trains (Shinkansen) every 20 minutes, with the journey taking about two and a half hours.
And if you're a real train-spotter, and want to take a long train journey, you can take the trans-Siberian in Moscow and get to the various points in which you can take a ferry to Japan.
Although perhaps it's not particularly useful, there are many maritime connections between Japan, China and Korea.