When do you use katakana? How do you know that?

We, Japanese use 3 types of letters like ひらがな, カタカナ and 漢字.
We use them mixing up.

But, When do we use katakana?

It’s used the one comes from foreign countries.

For example…

アイスクリーム comes from “icecream” which is English.
So normally we prefer use カタカナ.

ヘッドホン= Headphone
ミーティング = Meeting
ピンク = Pink　
*we have an another way to call, 桃色（ももいろ）which is Japanese name.
桃（もも）is “peach” in English.　
So, in this case “桃色（ももいろ）” is shown by ひらがな not カタカナ.
カーラジオ = car radio
マクドナルド = McDonald’s
スタバ = スターバックス = starbucks
アップルウォッチ = apple watch
フルーツ = fruit
….

カタカナEnglish

There are so many.
And It’s also used in onomatopoeia.
It is not all of them, but there are many like…

キラキラ spark
ワクワク excited
ドキドキ excited / heart beat
フラフラ dizzy
ペラペラ thin / fluently
ポイポイ through away
ポカポカ warm
….

Onomatopoeia

Your name would be written in カタカナ if you are non-Japanese

When you can see the textbook, non-Japanese name would be written in カタカナ. It’s because the person came from a foreign country.

I haven’t met the person who had a カタカナname in the Japanese.
If He/She is 芸能人（げいのうじん）, They might have a カタカナnama though.

Most of the Japanese have 漢字name.
Each 漢字 have a unique meaning.
When a baby was born, parents present their wish through the name.

My name is TOMO. 漢字 is 友.
This 友 means friend in English.
So, my parents wished had a lot of friends when I have grown up.

ひらがな and カタカナ haven’t meanings.
* Technical speaking, they have. But normally they don’t.

When you ask the name of the person who you met the first time.
You can ask the 漢字 as well. You could be closer friend with him/her, maybe.