katakana is used only for foreign words or sound words like "pow" or "smash". hiragana can used for everything else. Kanji can be used in place of hiragana to increase pace of reading and is used often. For example in japanese ice cream is アイスクリーム (aisukurimu) and since is a borrowed word or not orginally japanese you would use katakana. たべもの (tabemono) means food and since it is a japanses word you would use hiragana or as you progress and learn you could type 食べ物 which is the same thing as the hiragana but using some kanji. the kanji for eat (食） and the kanji for thing (物） literally translated to eating thing. Hope this helped you.

katakana is like this:ビイル hiragana is like this:こんにちは kanji is like this:勉強 I hope my answer can help you^^