(Ah, how nostalgic. It’s been three years since I stopped learning Japanese. Tbh, that sentence also took me a while to think up… :p)
I’ve been asked this question a few times, so I thought I’d just post it up here.
The alphabet: Hiragana and Katakana
I just searched “learn hiragana”. These links have sidelinks going to katakana, too.
Tofugu has a pretty good guide: https://www.tofugu.com/japanese/learn-hiragana/
A more traditional one: http://www.guidetojapanese.org/learn/grammar/hiragana
The fun stuff
A structured website to learn from: http://thejapanesepage.com/grammar.htm
The JLPT, or the Japanese Language Proficiency Test, is a good place to start for the basic grammar and vocabulary stuff. You can find a list here:
N5 is the easiest level and N1 is the hardest.
Also, Tumblr has many, many masterposts too for languages. Some would include Japanese as well. Just do an internet search, like so: https://www.ecosia.org/search?q=languages+masterpost+tumblr
For books, I’m good friends with a librarian at a Japanese library in Singapore, so you guys can ask me. Otherwise: http://search.nlb.gov.sg/Search?query=japanese&Language=Japanese
Know what your goals for learning the language are. Do you just want to speak it and be able to listen to it, or do you want to also read and write? Adjust your time doing each activity depending on which you want to build up more. Especially for speaking, you’d need to spend more time talking and learning about the more colloquial stuff etc. and if you just intend to speak it, you can skip some reading and writing parts. I like Fluent in 3 Months!
On time management: a teacher may be worth it to lock in the time and effort to do Japanese lessons so that it actually happens.
Look for ways to use language in your daily life, too. For example, my browser is in Japanese.
Practice hard, and good luck!