I'm not clear what you're trying to do...
The usual way to get Japanese input is to install software called IME. If you're using Windows, the IME comes bundled and all you need to do is activate it. If you're using Linux you install ibus and anthy. I don't know what you do if you're using a Mac, but it's probably similar to Windows.
Once the IME is running, you can switch from normal English input to Japanese input. You type Japanese in roumaji and the IME converts it to hiragana and, optionally, to kanji. So, for example, you type
nihongo and it will display as にほんご or, if you want, as 日本語. Incidentally, if you can see those hiragana and in kanji characters, your computer
does have Japanese fonts installed (alternatively, if you just see boxes or question marks the fonts aren't installed).
What kind of computer do you have? Is it running Windows? If so, which version (XP, Vista, Windows 7 or Windows
?
マイケル