No doubt this is probably why many of you are here. There are a plethora of guides on this topic all over the internet, many of which are written by individuals who cannot even hold a basic conversation in Japanese. You can rest assured that this guide is written by someone who lives in Japan and uses Japanese on a daily basis both in their career and their personal life.
Please read the entire guide before you begin studying.
Japanese uses 3 writing systems: Hiragana, katakana, and kanji. I understand that this is intimidating, but you're going to have to learn all 3 of them. It sounds scary at first (Believe me, I know!) but it's actually not so bad. There are a lot of claims on the internet that learning to read Japanese characters (especially kanji) is very difficult or even impossible; I want you to know that those claims are simply not true. If you follow my advice, you can get through them easily.
Hiragana and katakana each consist of 46 characters. Kanji, on the other hand, consists of a few thousand. You can just forget about kanji for now, you don't need to worry about it yet. I will teach you a secret later which will make learning kanji extremely easy, so don't fret about there being so many of them. Seriously, trust me on this. For now you need to start with hiragana. This is sort of a pre-requisite before you an actually start learning Japanese, similar to how as a kid you learned your ABC's before you started learning how to write actual words and sentences.
I want to make this very clear: Learning hiragana is the hardest part of learning Japanese. That might sound like a joke to some of the more advanced learners who are struggling with kanji (most likely using something like Heisig's RTK or WaniKani or similar tools), but I am absolutely serious. The reason why it's the hardest is because it is probably the first time in your life that you have attempted learning a completely new script. But I promise you, if you can learn all 46 hiragana then you can learn everything else in this language.
There are many ways you can go about learning hiragana. There are workbooks, free videos on YouTube, and many apps that you can use. Ultimately it doesn't matter what resource you use for hiragana, just use the one that works for you. What I do recommend is that you take the time to learn how to handwrite hiragana, as it is essential.
This website is handy for testing your recognition abilities:
https://realkana.com/Once you have finished with hiragana, repeat again with katakana and then move on to the next step. You can also use the above website for testing your recognition of katakana.
Try to finish this within 2 weeks.
Once you have finished learning hiragana and katakana, you are ready to move on to the basics. This is the stage where you begin building a foundational knowledge of the Japanese language in preparation for the bigger stuff. There are a two different ways you could go about this, the choice is yours:
You can do either of the above options and both are perfectly fine. Personally, I would recommend a textbook simply because it is structured and is often checked by native speakers before being published. Many of the popular online grammar guides and vocabulary lists are written by low-level speakers and are not checked by natives - as a result, they all have mistakes. It is worth noting that the popular textbooks also have mistakes, but not as much as the grammar guides. But please, do not think I am trying to push you towards textbooks over grammar guides - The choice is truly yours. Both have mistakes (we will address those before moving on to the next step, don't worry), so use whatever works better for you, just try to get through it as fast as you can.
Develop a study schedule and plan ahead of time how much to study each day. Ideally you want to finish this quickly. This might take you a couple of months depending on your speed.
While you are studying grammar from your textbook or grammar guide, it is also essential to be studying vocabulary. There should be vocabulary lists inside of your textbook, otherwise if you're using a grammar guide you should look for a vocab list online. I strongly recommend that you use flash cards to learn these words. And I strongly recommend that you use the Anki flashcard application as it is the best. There are even many free flashcard decks available for Anki. Chances are someone has already made one for your textbook, so most likely you do not need to actually create cards at this point in time. Just download a free deck and import it.
WARNING: Unfortunately there are some individuals (Most notably, a certain YouTuber) who sell flash card decks to try and earn a quick buck from beginners. DO NOT buy any flash card decks, no matter who made them. Trust me, you don't need it - there are enough free options available.
I will write a guide in the future for how to properly utilize Anki.
During this period of establishing the foundational knowledge, you should also spend some time to watch content in Japanese and get a feel for the sound of the language. Turn off the subtitles and just train your ears - don't worry about not understanding anything. I recommend anime.
Once you have Finished your textbook or grammar guide, it is time to address the mistakes.
https://nihongokyoshi-net.com/jlpt-grammars/The above website is a grammar guide written by native speakers. Find the grammar points that you learned in your textbook or grammar guide, and study them again here in order to get a better feel for how they work. I would have recommended that you start with this guide, but it's hard to recommend for beginners because it is written completely in Japanese. Some of the grammar points do have English included, but not all of them do. Don't worry about reading the whole guide right now, just tackle the grammar points that you already studied.
This is optional, but if you'd like something extra I recommend reading the book Making Sense of Japanese: What the Textbooks Don't Tell You byJay Rubin. It's a colelction of essays where he discusses areas of Japanese that are often problematic for English speakers. But again, this is totally optional.
Once you have finished with the Nihongo Net guide (and optionally, Jay Rubin's book) you may proceed to the next step.
Once you have completed building your foundations, you are ready to move on to the fun part: learning from media for native speakers. This is the stage where most of your learning will happen, and this is also the stage where your rate of learning will (naturally) speed up.
You no longer need to use textbooks. In fact, I would advise against them. All you need to do is enjoy media in Japanese and anytime you find a word that you don't know, stop to make a flash card, and then continue. Review your flash cards in Anki daily. This probably sounds tedious, but with a proper setup you can create high quality flash cards with a single click! I will write a proper guide on how to do this, but it varies depending on the format of media that you are studying from so it will take some time. In the meantime, just know that it is accomplished through using the browser extension Yomichan with the Anki add-on Anki Connect. There are other guides online for how to set those up, so in the meantime I would recommend searching something like "How to mine from Yomichan" or similar. I will write proper guides on how to do this with various types of media, so check the Learning Japanese page to see those when they become available.
If you would like, you can continue studying grammar through Nihongo Net during this step, but it's not necessary. Personally I recommend using it as a reference and checking there when you run across a grammar point that you don't understand.
It is important to learn from a variety of media and a variety of genres. That way you will be exposed to various types of words, and various grammar forms. Additionally, different types of media help with different skills. Reading is the most important; because novels (Or visual novels) are dense with words, you get more return-on-investment (ROI) per hour spent reading compared to consuming other types of media. Reading will do the most to advance your level, but games or TV can be beneficial also. Television is good for training your ears and improving your listening skills, and television, video games or comic books are all great forms of immersion when you're too tired to read a book (or visual novel) but still want to spend time doing something in Japanese. Visual novels are great because you can greatly expand your vocab through reading while Simultaneously training your ears (if it's a fully-voiced game) - they're also extremely easy to mine from.
Be careful not to spend too much time within a specific genre because it will limit the scope of your vocabulary. But if you must stick within a single genre, try to do so with stories which are grounded in everyday life so that the vocabulary will still be useful.
Continue with this step until you reach fluency.
You might be thinking I forgot kanji. Don't worry, I didn't. Actually, you don't have to study kanji. If you bought something like James Heisig's book (or similar) you should do yourself a favor and just throw it away. If you paid for WaniKani, go delete your account. You don't need to study kanji.
Don't misunderstand me, kanji is absolutely essential for learning Japanese, but studying kanji in isolation will get you nowhere, and will do nothing but waste your time. Instead, just learn kanji in the context of the vocabulary that contain them. For example, you don't need to study all of the different ways to read 生, just learn words that contain it like 生きる and 学生 and that will teach you everything that you need to know. You don't need to study the arbitrary meaning of each character either, you will learn that through studying vocabulary also.
To put it simply, the only way to get good at conversation and speaking is to have conversations with native speakers. Make friends with native speakers and try to talk to them in Japanese. I recommend that you try to befriend people who can't speak English, that way they can't switch the conversation to English.
If you live in Japan, join a local sports club or something and you can easily make friends there. If you live outside of Japan, maybe consider hiring a native to talk with you on iTalki. You might be able to make friends on Hello Talk also but both of these methods are inferior to having actual native friends.
If you're having trouble with pronunciation, try shadowing. Watch videos on YouTube and try to repeat the spoken lines exactly how they're spoken. Find a YouTuber you like (who is the same gender as you) and try to speak like them.
Just like speaking, the only way to get better at writing is to write. You don't have to do this (personally I don't), but the easiest way to get better at writing is to keep a handwritten journal and write about various topics. Have a native speaker check it for you and make corrections, then rewrite it again using their corrections.
You should be working on kanji, speaking and writing throughout your entire learning journey. Do not wait until the end to start those.