The basically rule is that katakana should be used for the words and names which came to other languages except some. In the formal situation, we have to use kanji and hiragana which is called okurigana, 汚い. For small childrends, we use hiragana. However, in the casual situation, there aren't specific rules, especially, in anime, manga, text messages between close people, etc. Some use katakana to emfasize the words or other reasons, or simply they want to use katakana but we all know the situation where we shouldn't write in this way. So, I don't recommend the beginners to use katakana for the Japanese words.
I hope you are clear now.