To read and browse Japanese on Ubuntu Linux, you don't need to do anything. The Japanese fonts are already installed. To be able to enter Japanese text it's a little more tricky. On newer versions, Ibus is the way to enter text with languages other than English. On the web you'll see lots of references to Scim - don't use it. It's obsolete. Ibus should already be installed. If not, get it from the Ubuntu Software Center.
You will need "Anthy" input method for Japanese. Get that from the Ubuntu Software Center.
The next step is to configure Language support under System, Administration. Click Install/Remove Languages to add Japanese. Then go to "keyboard input method system", select "ibus", and close the window.
Now start Ibus. That's under System, Preferences, Ibus. It will prompt you to start the Daemon - you will do this every time you log in, unless you edit the .bashsc file as indicated. To do this, copy the text, paste it into Gedit, and save it as .bashsc in your home directory. Be careful that you aren't overwriting an existing .bashsc. If there's already one, add those lines to it.
Once you've started Ibus the Preferences dialog will appear. Click the Input Method tab and Select the Japanese input method "Anthy" (It's got a little crown icon). Click Add, then click Close.
Now open a text editor or browser and put the cursor on a text entry area. You will see a keyboard icon in the top right tool bar and a strange white square in the lower right of your desktop. Click the keyboard icon, and click the Japanese - Anthy from the drop-down menu. Now the strange white square will become a bar that shows the Anthy options. A hiragana あ is displayed so now you can type hiragana and kanji. Click the あ to select katakana when you need it.