[Pimpzilla]

It would be wise to us all three. Katakana is generally used for foreign words, and hiragana is used as a basis for pretty much everything else. Kanji generally takes the place of some of the hiragana in reading material (my sensei says that this is mainly for speed reading), but hiragana is combined with the kanji for normal words and as verb stems. An example of an ordinary word would be お守り (meaning 'charm' which utilizes both hiragana and kanji). An example of a verb would be 忘れる (meaning 'to forget' which uses the hiragana to conjugate the verb). It's a lot to put up with, but if you want to learn the language you will just have to make your peace with it.

*Eh, I just read all the posts and Hackimoto said pretty much what I had to say, sorry!*