This HOWTO will allow you to type in hiragana or katakana characters in to your forum posts or in to any other document. This was done in Mac OS 10.4 so may differ slightly in earlier versions.
First, open up your 'System Preferences' and select 'International' from the first row. Now select the 'Input Menu' tab. At the top you should see an item called 'Japanese Kana Palette'. Checkmark the box next to it to enable it. This will give you a "kana picker" that you can select individual characters from. If you would like this picker conveniently located, checkmark the box at the bottom called 'Show input menu in menu bar'.
If you do not see any of the above options you will need to add Japanese Language Support from your Mac OSX install discs.
After this is done, click the flag in the Menu Bar and select from the menu 'Show Japanese Kana Palette'. This will bring up your picker. Click the character you want to insert and away you go!
-ken
[edited by Jason to add]
For actually typing in Japanese characters
First, I strongly suggest checking "Show input menu in menu bar" as Ken already mentioned. I also recommend selecting "Use one input source in all documents" on the same tab. To be able to type in Japanese, from the Input Menu tab, check the box next to "Kotoeri." When you're ready to type in Japanese characters, click on the flag in the menu bar and chose hiragana. Typing and converting to Japanese works the same as in Windows, so read sections 6-14 and 16 at this link for details.
-Jason
And I don't have the desk space for an external one.
hey! shift ->katakana ? Great! 
but I can type real fast 