The easiest way to write Japanese on the Mac is to utilize the build-in Japanese keyboard, where you can enter words ‘phonetically’ and it maps them to words as you type, allowing you to select options if there are multiple.
The somewhat better option is Google IME, which is fairly popular and generally has better suggestions.
Whichever one you use, the biggest hassle for people who write another language besides Japanese is to switch between the keyboard layouts efficiently. On actual Japanese keyboards you can use a key to switch quickly between latin based input and kana/phonetic input. On the Mac you can select a similar option in your keyboard shortcuts to quickly switch between layouts. The choice of a shortcut, however, can be somewhat difficult if you think about it.
A nicer option in my opinion is to use KeyRemap4MacBook, which allows you to reprogram certain keys to the task that you normally can’t use that way. An obvious choice would be caps lock, which few people really care too much for. Since I already remapped it to be a control/escape combo in my setup, I chose the right command key to be my keyboard switch. For shortcuts I really only use the left one anyway, so the right one is just never used anyway. Plus it’s very easily accessible.
But hey, KeyRemap4MacBook even allows for combo mode (which I used for caps lock lock as well): If you press the key with a shortcut, it works like normal. If you press it by itself, it works as a keyboard switch. Just search for “command_r” in the settings and look for the KANA option.