the majority of Japanese I learned in highschool for 3 years and then self-studied after that. I honestly always found Japanese an absurdly easy language to get the hang of, it's the stuff that's attached to it that's difficult (i.e. kanji, social and cultural usages of the language, etc). Japanese as a language is ironically pretty simple and straightforward, since it literally grammatically categorizes itself in its own usage via the particles. However its simplicity means that what's unsaid or what's subtext is far more important, ergo the cultural research.

All that said the major tip I have is to focus on the writing system early, frequently and often. Crunch hiragana/katakana as fast as possible (my teacher spedrun us within 2 weeks) so that you can read/use it as often as possible, and get in the habit of learning 1-2 kanji a week at least. Luckily there's a lot of kanji workbooks that can help you out with that.

As you focus on that, verb forms, number labelers (or counters, like how to say 'one book' or 'three peaches) are next, and then particles and its usage in various sentence forms. You start shifting from simple sentences to the complex as you go along with that.

And immersion is always important. Start challenging yourself by reading Japanese without instantly looking at the translator (might need some furigana plugin to do so to be able to read kanji as you're learning, at least online) or by watching anime/dramas without subtitles and see how much you put together.

And: practice, practice, practice. Use it or lose it.