Katakana is typically the second Japanese script that you will learn. Every hiragana “letter” has a katakana equivalent.
Katakana is primarily used for the following purposes:
Writing loanwords – When Japanese doesn’t have a word for something, it typically will just import the word from another language (usually English) and write that word in katakana using Japanese phonetics. For example:
America – アメリカ (amerika)
Ice cream – アイスクリーム (aisu kuriimu)
Part-time job – アルバイト (arubaito – from the German word “Arbeit” meaning “job/chore/labor”)
Foreign names – If your last name is Smith and you go to Japan, there’s obviously not going to be a Japanese word for Smith, so your name gets written in the closest possible approximation: スミス (sumisu).
Onomatopoeia – Especially in Japanese comics (manga), sound effect words are written in katakana. For example, a dog barking in Japanese makes the sound “wan wan” which is written like this: ワンワン
Technical terms and company names – This is common, but not always set in stone.
After learning hiragana, your next task is to learn the katakana as quickly as possible. The sooner you learn them, the sooner you can start reading Japanese words and not having to rely on romaji transliterations.
Another thing you will notice when it comes to katakana is that it is used on product packaging a lot, perhaps as a sound effect that a toy is making, or in place of many English words that are just being written phonetically.
A trip to a bookstore or toy store will often give you lots of practice when it comes to reading, and when you’re a beginner, identifying a word that is actually English will be a great confidence booster. Imagine being at a restaurant and seeing アイスクリーム written on the menu. As soon as you learn to read, you will automatically know this word without having to memorize any vocabulary or anything! Plus, ice cream is delicious so you will know what to order, as well!