Reading raw manga/doujinshi helped me learn slang(not the hentai ones because those contain repetitive words).
Slang helps if you wanna fit in. haha.
Nihongo sou matome is the best book.
Find a company that will sponsor your education and exam fees.
丁寧語is necessary at work. Officemates just teach me along the way but reading specific material on it helps too
For Kanji, Many books in Japanese having ruby furigana next to kanji. So first you have to learn hiragana & katakana.
Don't worry, many Japanese people like myself cannot spell all standard 2000 kanji anyway and depending on
computer convert automatically.(笑）
There should be web class for learning Japanese...people get on that! If there is oh link it up!
Memrise has been pretty good so far, but I think once you get to a certain point you have to pay the full price for the content which kinda sucks.