This is how I learned it:
Hiragana: used for particles, adjective/verb endings, words that don't have a kanji, etc. (Also, if you don't know the specific kanji that is supposed to be sued, you can use Hiragana instead.)
いい 楽しい 帰る
Katakana: used for foreign words.
オレンジ アルバイト チーズ
Kanji: characters borrowed from chinese. used to write like nouns, adjectives, verbs, and what not. Not all words have a kanji, and if you don't know the kanji, you can write it in Hiragana.
空 話す 忙しい
Hope this helps