Hi! You can look at Hiragana as like the "ground level" of the Japanese writing system, as it's what kids learn first. Katakana is specifically for non-Japanese loanwords (like chocolate, which doesn't have a Japanese equivalent so they say "chokoreeto") and Kanji are basically used to make reading quicker- you could write the word for "I" like "わたし”, but it's quicker to read and write ”私”which is the kanji for it. You need to learn about 2000 Kanji to read a newspaper. They also can change their pronunciation depending on which word they're being used in. Hope this helped a little! :)

Hiragana: These are the building blocks of suffixes, grammatical particles, and sometimes base words. They're generally the second most common script behind kanj. Katakana: Primarily used for loanwords and, in my experience, for writing animal, plant, insect, etc. names without kanji. They might also be written with hiragana too though. When you see katakana words you should try pronouncing it in your head then try to figure out which English word it's trying to approximate, since a large majority of loanwords are from English. This is not always the case, but it's a good rule of thumb to follow. Keep in mind Japanese does not use articles (a, an, the, etc.) so if you see a word like アパート (apaato) you should read it as one single word, not ア・パート。 アパート means apartment while ア・パート would mean a part (but remember, they almost never use articles even for loanwords, so "an apartment" is just アパート。) Kanji: The meat and potatoes of the Japanese language which are distinct from other scripts because of their complexity (in design, not necessarily in readability or your ability to remember them) which help separate words and actually helps readability. Here's an example sentence that uses all 3 scripts. Note the difference between kana suffixes (しい、る、った) and grammar particles (と、に、を、に) and how when combined with kanji makes it clear where words start and end. Going all-kanji or all-kana makes it much harder to tell otherwise. 一昨日、彼氏と一緒に新しいトランスフォーマー映画を見るに行った You need to learn all 3 scripts to understand the written language. As with any language, of course you don't need to learn them for the SPOKEN language, but being illiterate is very difficult in this day and age.