A friend is moving to Japan for a few months and speaks no Japanese - thought I'd share some tips I gave them in case it's helpful to others.
Figure out why you're learning Japanese. If it's just to make friends for a short stint in Japan, focus more on conversational practice. If you want to read native Japanese material, spend more time on kanji and grammar. If you want to have a good time travelling in Japan, focus on things like asking for directions, words that pop up in restaurants and subways etc. Detail out your goals granularly so you can measure progress, feel motivated when using your Japanese, and can downscope how much you have to learn.
Don't spend too much time researching optimal resources or study plans. It's best to get started immediately using some of the basic resources mentioned below and try get to a consistent daily habit. It's great that Japanese has a huge online community of learners and materials, but often you see people overthinking how to learn Japanese, and not actually learning Japanese. Tofugu has an opinionated study guide and list of resources, and there's also a detailed wiki from the /learnjapanese subreddit.
Speedrun hiragana and katakana: Move away from using romaji as soon as you can - try and learn how to read and pronounce hiragana and katakana in a weekend, as well as how to type hiragana and katakana on your phone and laptop. The first section of Tofugu's guide lays out some good context and resources - just start there. You might want to spend less time on kanji using Wanikani (Tofugu makes Wanikani, the spaced repetition software for learning Kanji via radicals). Tofugu recommends learning 300 kanji before moving on, but I think you could probably start using a beginner textbook while using Wanikani for kanji if you like the platform.
Find a good beginner textbook - work through it while practicing conversation. I really liked the Genki I and II textbooks because it focuses a lot on useful conversational Japanese, and includes grammar, vocabulary and kanji help. I just did that and a lot of conversational practice before I arrived in Japan, and it gave me enough to start making friends and having fun in Japanese.
Start speaking Japanese earlier than you feel comfortable. When I first arrived, I focused more on vocabulary and using that vocabulary in conversation more than anything else - it served my goals of making friends and getting around Japan. I used italki to book in lessons with tutors, solely focused on conversational practice (doing all the grammar / vocab / kanji work on my own time). There are lots of language exchanges in Tokyo, which are nice for making friends, but can be a little inefficient - if your Japanese is still rough and your language exchange partner has decent English, you'll find yourself defaulting to English in conversations; you have to be quite adamant about having a decent split between English / Japanese, and not everyone is a great teacher. Try and get as many varied reps in each day as possible - ask people for directions, make small talk, practice introducing yourself etc.
Shift as many of your daily activities over to Japanese as you can. Watch some Japanese Youtubers; use extensions to see both English and Japanese subtitles in Netflix (and watch kids' movies or reality TV with Japanese audio). Read NHK Easy. Use rikaikun to show the reading and English definition of Japanese words when you hover over Japanese text in the browser. Use imiwa on your phone.
Anyways that should get you started. Good luck!
