This is a great introduction of Japanese language:
Japanese writing system composed of three systems: Hiragana, Katakana, and Kanji (Chinese characters).
You should learn Hiragana and Katakana first, do not work with romaji because no one uses it.
Learn as you go. It is hard. Try to associate with something to remember what does it mean. Do not need to learn how to write at the beginning.
Easy Japanese by NHK, released 2019. 48 units of intro Japanese that help you get started.
Genki by Japan Times is the most popular textbooks, but I would say it is hard to self-study without a good tutor.
Video is a better form of study for me. I use Language Learning with Netflix to watch Terrace House on Netflix that builds up a lot of phrases, the sense of language, also improves a lot for my listening. Ainori Love Wagon: Asian Journey is also amazing. Also see comprehensible inputs for Japanese.
24 grammars: http://www.coelang.tufs.ac.jp/mt/ja/gmod/courses/c01/
48 grammars: http://www.coelang.tufs.ac.jp/mt/ja/gmod/courses/c02/