There's an app called Kanji Study for studying Kanji. It should be noted there is a paywall in this one, but the early parts are completely free.
There's also Anki, which is a popular choice. I never used it much, though.
I know Wanikani is popular, but I've never tried it. It might be worth looking into.
Duolingo is a popular choice, but having completed their Japanese course recently I can't really recommend it in its current state. I guess it's better as a supplement.
There's also minato, which is completely free and is kinda indirectly linked to the Japanese government, so it's the most "official" method though it's not very popular. It has some interesting courses like ones specializing in certain dialecs of Japanese. They also have a placement test so you can know where to start at in their courses.
