Determine whether you need to read and write Japanese or simply speak the language. Learning to read and write Japanese is more difficult than simply learning to speak because the character system is so unfamiliar to English speakers. If you choose to learn all of these skills, you will not be able to progress as far with your speaking skill.
Purchase a computer program for learning Japanese, such as Michel Thomas, Rosetta Stone or Pimsleur. The FBI often uses Pimsleur to teach its agents languages when they need to learn very quickly. It is a 20 hour course, Michel Thomas is an eight hour course, and Rosetta Stone offers various packages which can provide more than 10 days of instruction.
Find Japanese podcasts, TV shows, movies, newspapers or books. The more exposure you have to Japanese, the faster you will learn, even if you do not always understand everything.
Devote as many hours per day as you can to learning Japanese. Spend at least six hours with your computer program, and spend the rest of your time listening to or reading Japanese media. If you are truly determined to learn Japanese in 10 days, you must spend as much time as possible immersing yourself in the language.
Move quickly through your computer program, even if you do not grasp everything perfectly. You will pick up what you miss as you continue, but if you stop and focus on getting everything perfect before you continue, you will not make it very far in 10 days. Wrap your review into your new material so that you reinforce ground you already covered while you progress, thus allowing you to move more rapidly.
Search for a Japanese conversation group in your area or hire a tutor so that you can practice having a real conversation and ensure that you pronounce the language correctly. Speaking pre-set phrases to a computer on your own is much easier than engaging in real-time dialogue, and the immersion will encourage your brain to pick up new words and phrases much faster.