Here is some information on how to set up your computer to use Japanese characters.
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How to set up Your Operating System for Japanese
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Various Operating Systems
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How to Install a Japanese Keyboard – On Apple, Windows, Computers, Phones, You name it! (From Tofugu)
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Windows
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Installing IME and Typing Japanese on Windows (From CosCom)
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Mac
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Using Japanese on a Mac (From Japanese for Your Mac)
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How to Type Japanese Characters Using English Keyboards
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Japanese Typing Practice for Beginners (From University of Washington)
This is a little old site, but the actual typing practice is quite good with useful tips on how to type Japanese character using Roma-ji.
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