If you have minimal experience speaking, reading or listening to the Japanese language then it can be intimidating to try and communicate with the native Japanese speakers around you. While apps can help, if you can learn the language while on JET it will enhance your experience.

I have regretted not taking Japanese when I was in high school (I took French instead and dropped it after the first year) as it is much easier to relearn than to begin to learn new language as an adult. Do not fear, you are capable and having a goal, such as aiming to complete N5 (or higher), will help retain your motivation.
My personal goal was focused around reading Japanese as I felt that was my weakest area. Watching anime has helped my listening skills and, since Japanese and Māori are both phonetic languages, I feel that my pronunciation is passable. I began with Hiragana followed by Katakana and found that Daiso was the best place to get resources to learn to read and write the characters. The books are aimed at pre-school kids, but since that was my level it was perfect. There were images of the characters, spaces to practise the stroke order and words on the side that matched up with the colouring-in pictures on each page. I got a pack of felt pens at the same time and was able to use the books multiple times by starting with the palest blue colour pen and then writing over the top of my previous attempts in darker and darker blues so I could see my progress.
My next step was to increase my reading speed so I began testing myself using Quizlet. That website has online flash cards, tests and games to drill yourself.
Hiragana set: https://quizlet.com/34289851/hiragana-flash-cards/
Katakana set: https://quizlet.com/82323494/katakana-flash-cards/
Hiragana and Katakana set: https://quizlet.com/143785116/hiragana-katakana-flash-cards/
Comparing Hiragana and Katakana: https://quizlet.com/vn/831047190/hiragana-katakana-flash-cards/

Then I got ambitious and began a new game in Pokémon Y with the Japanese Language setting (no Kanji) and began a nuzlock. I read the sentences that combined hiragana and katakana then put them through Google Translate as I couldn’t pick up on most of the language.
Kanji was next and I went back to Daiso to get a book with the first 100 kanji and decided that I would aim to learn the kanji step by step as taught by NCEA (the Secondary School Education Qualification). I’d like to be able to talk about my progress but here is where I hit the kanji wall. Learning the different pronunciations of kanji doesn’t work like the other characters because a lot more context is required to parse out the meaning.
If you want to start on Kanji then here is the Quizlet I used for the NCEA Level 1 Kanji: https://quizlet.com/234097230/ncea-level-1-kanji-flash-cards/

To improve my vocabulary and learn about sentence structure and grammar I used MaruMori. While DuoLingo is often an easy place to start for app or online learning I got frustrated with it pretty quickly as it was memorisation without reasoning. I didn’t know why I was learning a particular phrase or how to know what particle was required. MaruMori is the opposite of that. There are dense paragraphs of explanations with plenty of examples of both correct and incorrect grammar usage and drills using the vocabulary you just learnt. The paragraphs are also written in a conversational format, so it doesn’t feel stiff or stuffy and the whole website is cute.
Disclaimer time: you do have to pay to get past the beginner’s section but if you are serious and find that this website suits your learning style then it is worth it.
I am no longer putting in the same amount of time to learn Japanese during my desk warming time as I did last semester but I would say that it is one of the best ways to spend that time while you are on JET, especially if you plan on staying for several years.