If you research options on living in Japan, you might sometimes find comments that go something like this “you do not need to know Japanese to live in Japan, just be an English teacher and you’ll be fine”. Sure, if you are coming to Japan as a tourist, you will probably be fine communicating in English or with your hands and feet, but if you want to LIVE in Japan, I cannot recommend learning the Language enough. When in Rome, talk as the Romans do and all that. Just even a basic greeting in Japanese will open many doors that would have otherwise been closed to you and it takes so little effort to learn a few basic phrases that there is no excuse.
Now, do not get me wrong, Japanese is a very, very difficult language, especially for people from the western hemisphere, and even being able to have daily conversations is considered a pretty big achievement. Becoming fluent is something that is going to take most people years of study and effort. The motivation for starting to study can be different, some people study so they can understand Anime or Japanese videogames, others do it because of their interest in Japanese history or love for Haruki Murakami books. But in my experience, this motivation is not enough to keep you going until you truly reach a fluent level, close to a native speaker. If you want to make it that far, more often than not you are going to have to put yourself in situations where you have to speak Japanese, force yourself to adapt to your surroundings and advance your language skills. There is a decently sized English community in Japan (especially Tokyo and Osaka), and it can be very tempting to always fall back on these communities for everything you need (be that simply making friends, or things like medical assistance, getting a haircut etc.). But by doing this, you are isolating yourself and denying yourself a chance to challenge yourself and grow as a person. I am not saying that everyone should be able to speak perfect Japanese, but you owe it to yourself and your surroundings to try to get to a conversational level at least. If you manage to do that, your time in Japan will most likely feel more fulfilling.
Now, as for how to study the language, in the digital age, there are hundreds of different options available. If you have the time, then attending university classes or a language school is a good idea, especially if you are a total beginner. If that is not an option, there are countless books, YouTube channels, websites and apps that teach Japanese (of note here is, that most of the available options will most likely be English/Japanese). One big trap that many people fall into, is the search for the one-stop learning solution. That ONE book, ONE channel, ONE app that teaches you everything from basics to fluency simply does not exist. Studying is an ongoing process, and that includes the continuous search for appropriate study material. Another mistake is not using the language skills that you acquire during your studies. Japanese is not a language that is as widely studied as English, therefor a lot less study material targeting language learners exists (e.g. novels aimed at non-native speakers with a side-by-side translation etc.). Material is available, but you might have to search for a while. I recommend getting a SRS (spaced repetition) application such as Anki, which is really just a fancy name for digital flashcards and using that as you study. Other recommendations would be WaniKani for Kanji and NHK Easy News to get used to reading in Japanese.
Accept that this is going to be a struggle, but it is going to enrich your life, grant you access to a completely different culture. At the same time, it is also going to make your life in Japan (if you want that) a lot easier and make you more attractive on the Japanese job market as well. A more detailed guide on studying might come in the future, for now, I am going to leave you with this rant