I'm actually a language teacher and worked at a linguistic company as an Army Contractor teaching people how to learn languages quickly. I've mastered Spanish in about a year (it's actually one of the easier languages) and I can give you some helpful advice.

Inundate yourself in the language. Watch, listen and speak as much as you possibly can.

1. Vocabulary. Even if you don't have the right order, most people will understand.

For instance: I go to the store in the past. (Most English speakers would understand that the person WENT to the store even though this person didn't know.)

How to build your vocabulary: Japanese has a distinct writing style, but they do have a Romanized version of their language so if you're merely looking to converse: Watch (a) show/movie/content in Japanese and have the subtitles if at all available with Japanese (Romanized) subtitles. A LOT of music is like this.

For every word you don't know, write it down and translate it later.

2. Grammar. This is the tough part, but you'll find that English grammar is more difficult than most languages because of rules but especially for exceptions. I'm not an expert in Japanese, but it will probably be more difficult.

Best way to learn grammar: KNOW YOUR OWN LANGUAGE.

Yes, having a grasp of your own language makes it so much easier to learn other languages. There are tons of exercises for grammar, but if you don't understand the basics of English you will have difficulty learning another language.

3. Practice! Every day, every way, all the time. Talk to yourself, talk to a friend, make speeches. People love to teach, and if your girlfriend and/or coworkers speak Japanese -- practice with them.

I hope this helps.