Konnichiwa (=Hello)!
Look like this, you have a Linux computer, and you were on Japanese forums in your Windows time.
After your Linux instalation, you can't typ Hiragana, Katakana and Kanji anymore.
Non-sence!
Here I'll explame how to do that.

First of all, install a Linux distro that supports SCIM, like Ubuntu 9.04 or Linux Mint 7.
Next, install the Japanese packages, log out, select Japanese language, click "No", and choose "For this section only", if you don't want to lose your default.

Click Menu, Applications, or whatever you have.

Choose Tekisuto Edita (=Text Editor).

On the right top/buttom corner, where you will see a keyboard, choose Nihongo (=Japanese), or Anthy, if you can't read Kanji, but the pop-up in Romaji.

Here you are! :-)

Japanese isn't that hard.
SCIM can come in very handy if you want to write on a Japanese forum.
And on the side fact.
With space you can change a word, in case it needs to be Kanji, Katakana, or something else, and with enter you can typ further.