how do you know when to use hiragana or katakana ?
and do we use kanji to only write less ?

See a translation

katakana are for foreign words that are not Japanese, such as ringo (apple) and pen

hiragana is used widely, and is the fundamentals of Japanese. It is used in both Japanese grammar and vocabulary. With just hiragana, you would be able to read even the most difficult kanji if they contain furigana (small hiragana wording on top of the kanji to help you read. Mostly used for children books, manga, or difficult kanji and names)

There are Japanese words with the same reading, hence if you write hiragana passages, it is difficult to read and the reader may have to guess the meanings if they are not Japanese experts.

e.g. 当たり and 辺り. Both have the same reading あたり (atari)