I've been majoring in Japanese in college for 3 years now. Honestly, speaking and actually
using the language are the most important things. But if you're looking for something to study on your own, I recommend finding some way to cram vocabulary. Forget grammar and idioms, you'll need a teacher and/or immersion for that. Half of the entire ball game is simply how many words you know. It doesn't matter how simple the sentence is, if you don't know a crucial word, you aren't going to be able to say it.
Before I started taking classes I bought a CD-ROM set called "Instant Immersion Japanese". Like most teaching software it didn't get me far, but it did include a program called JQuicktrans, which is really a glorified dictionary. It's got a study list feature with flashcards, though, and I use it a lot. If you like, I can send you this program.
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