[mira]

Romaji are Japanese words written with English letters. It's not really used in Japanese language.
Example of Romaji: Houyouryoku

Hiragana is the Japanese alphabet. Words that have no Kanji, particles, and other things are written in Hiragana. There are 46 Hiragana characters plus things called "smalls". Sometimes you will see small Hiragana characters above Kanji, so you can see how it is pronounced. this is called Furigana. Hiragan is very important because it is the Japanese alphabet. I really suggest you learn this one first. You need to learn and practice exactly how to write it and pronounce each one.
Example of Hiragana: つづきます

Katakana is Japanese characters used to write foreign words. If you were to write your name in Japanese it would be in Katakana. It's also used for onomonopeia or to kind of make something look fancy. This is the second alphabet you should learn after Hiragana. You need to learn and practice exactly how to write it and pronounce each one as well.
Example of Katakana: スピード

Kanji are characters borrowed from Chinese which can have different readings and meanings. Japanese even have a few of their own. Each Kanji has an "on" reading which is from Chinese and a "kun" reading which is Japanese. Kanji is the hardest one because there are about 2,00 Kanji that Japanese learn each with multiple readings. Even native Japanese speakers aren't able to read all Kanji until high school. Kanji is really important if you want to be able to read things in Japanese, and if you stay committed you will learn! It's important to learn the stroke order, number of strokes and both types of readings.
Example of Kanji: 震動

Good luck! And don't give up. Japanese can seem overwhelming a lot of times but if you try hard and practice a lot you will learn!