OSX has Japanese language support included. Either you can run the entire system in Japanese (Good Luck!) or have the ability to write in kana and kanji and view Japanese documents.
Using the International System Preference you can use Romaji, Hiragana and Katakana using a qwerty keyboard.
Use a program that has support for Unicode. TextEdit does for instance. Use a Japanese font (Osaka or Hiragino Kaku Pro).
Select the character system you want to use. Type in romaji (konnichiha) and it will convert on the fly to hiragana (こんにちは) or katakana depending on which keyboard layout you’ve selected. The system also deals with double constants properly and the various や, ゆ, よ, combinations. Press enter when you are happy with the word you have written, then press space if you are separating the words. (Japanese only seems to have spaces after sentences. However this makes more sense when kanji are used and I find that a large run of hiragana difficult to read but maybe thats because I’m used to english.) You can also convert to kanji if you know the correct kanji to choose from a pop up menu that appears when you press space without pressing enter.
Shift when using hiragana will change to katakana. CMD-Space toggles between the last two character sets used. CMD-Opt-Space cycles through the keyboard layouts selected in International. Remembering this stops you merrily typing away in English and having it converted phonetically to Hiragana.
There is also a floating pallette that allows you to select characters by clicking on buttons.
Unfortunately the help for Kotoeri input is in Japanese as it’s intended for speakers rather than learners. However the manual for the previous release of Kotoeri is still available as a download from Apple. Other help might be found at The Multilingual Mac.