The best way in may experience (I learned Japanese for about 8 or 9 years culminating in a year abroad there to study it intensively) is to be around native speakers. It's great to find other people learning to help, but you may find that you both end up supporting bad habits in each other you don't realize you started.
Ultimately, I find there is a great line between being intermediately good in Japanese, and being truly advanced. I'm not sure I ever crossed that line, it takes intense dedication, and really has to be your primary interest. The problem is that Japanese is not a single-use language, it's basically comprised of multiple styles, or "sub-languages" I guess, that each get used in very particular situations (speaking to friends sounds much different than speaking to a boss, down to the words themselves). It's incredibly hard to juggle in your head, and it's something even native speakers have trouble with.
I hope you can, though! Just remember that thanks to those different forms of Japanese and Kanji, Japanese takes far longer to truly get good at than most other languages, especially European ones which share commonalities. Luckily you'll have a pronunciation advantage, as it's pretty easy for most Western-language speakers to pronounce Japanese sounds.
If you want textbook recommendations, for beginners you can't get much better than Genki.
