This guide is meant to be simple and to the point. I am currently at an intermediate level in Japanese. I do not practice speaking so that will be the limitation of my knowledge but this should help you skip the exploration stage of having to inundate yourself with information to get started.
There will be a mixture of paid and free resources throughout this. You do not have to use the paid resources—I only share them as I have found them valuable and worth the money.
Japanese has three writing systems. Hiragana, Katakana, and Kanji. There is Romaji as well, though it technically is not a writing system.
Oversimplified explanations:
Hiragana=Native Japanese
Katakana=Characters used for foreign syllables
Kanji=Chinese characters used in Japanese (over 10k of these)
Romaji=Japanese using the English alphabet
I encourage you to start with the basics. Learn Hiragana and Katakana. Don’t focus on writing unless you want to. With repetition through quizzing and reading, you will learn these characters.
While Romaji can be a very helpful support system when learning (especially when it is used as furigana—the small text above characters), I strongly discourage leaning on this as your brain will default to reading Romaji since it is easier/more familiar.
My favorite resources will predominantly come from Tofugu. They have a phenomenal library of free tools. Explore as you wish but remember that practice is what ultimately will help you get better.