What is the fastest, easiest way for me and my Japanese girlfriend to get married and be able to live in the States?

It depends on whether she is already in the US or still in Japan, whether you plan to marry in the US or in Japan.

Additional information is required. The fastest easiest way to achieve the desired result depends ultimately on facts and circumstances peculiar to you and your fiance, including but not limited to you and your fiance's present physical location, your fiance's immigration history if any and a host of other matters. Getting residency in US requires a detailed review of a number of issues that is best addressed first by consulting with an experienced and well recommended immigration attorney who can assist you in sidestepping the various minefields in the process. So, the fastest means must include one where you and your fiance has consulted with an attorney. Otherwise, mistakes might occur that would result in undue and avoidable delays.

You can marry in the US and apply for the green card. The process takes about 90 days from start to finish. Just make sure to wait at least 30 to 60 days after her last entry before getting married. Otherwise, you can have issues getting the green card.

If you are a U.S. Citizen, you may apply for K-1 if you have met her in person at least once in the past 2 years.