[ChiAmaterasuNeko]

Hiragana is used for native Japanese words, whereas Katakana is used for words taken from English. For example. English "milk" translates to "miruku" in Japanese. A word like this would be written in Katakana. If I'm not mistaken, Katakana can also be used for emphasis. So in writing, they are used together.

With regards to Kanji, small hiragana is sometimes written above the Kanji. The smaller characters are called Furigana and are translations provided of the kanji, often for younger readers.

Even if I didn't answer your question exactly, I hope it helped. ^^