Apple operates different iTunes Stores for each country. You will need an Apple account for each country's store that you want to use. For example, if you are a U.S. resident who loves Japanese music, you will probably want both a U.S. account and a Japanese account.
You need to register each Apple account with a different e-mail address. Also, usually, you will add credit card information to each account; however, you can only add a credit card from the country of the account (for example, only Japanese credit cards for the Japan store). So, if you do not have a credit card with a Japanese address, you will need to purchase iTunes gift cards (which
you can buy from me).
How do I get a Japanese Apple account?
See
my guide.
How do I actually use both accounts?
An Apple device can only be logged in to one Apple account at a time. Since Apple now has iCloud, which is a package of software and features for automatically syncing data between your Mac and your iPhone/iPad, you most likely want to use the account that you use on your Mac on your iPhone/iPad as well, because this will allow you to automatically sync data back and forth.
To purchase apps, books, movies, and music from the primary account, just open the App Store application on the iPad or iPhone and make your purchases.
To purchase from other Apple accounts and add those purchases to the device, you will need to make the purchases in iTunes on your computer and then sync those purchases over to the iDevice.
Do I need to buy Japanese apps from the Japanese Apple iTunes store?
If an app is not available for purchase in the U.S. Apple iTunes store, you will need to purchase it from the Japanese Apple iTunes store. However, such apps are few and far between. Most apps are available for purchase in both the U.S. and Japan stores. Apps automatically include all existing translations regardless of the store. So, there is no "U.S. version" or "Japanese version" of apps. If you can purchase an app from the U.S. store, it will automatically appear in Japanese if your iPhone, iPad, or iPod is set to the Japanese language.
Generally speaking, it is a better deal to purchase from the U.S. store when you can. However, there are a number of free Japanese apps that offer in-App purchases which require purchases from the Japanese store.
Do I need to buy Japanese music, movies, and books from the Japanese Apple iTunes store?
Yes, very few Japanese songs, movies, or books are available outside Japan. Purchasing these will require a Japanese credit card or an iTunes Japan code (which
you can buy from me).