How to learn Japanese: First, get a book and master the Hiragana and Katakana syllabary. This is a must and it shouldn't take you more than a month to do that. Japanese for Busy People has a Kana workbook and that works very well with this first volume since it includes vocabulary from chapters 1 through 10. That way, by the time you immerse yourself with this book, you can start focusing more on sentence patterns. Once you've familiarized yourself with the Kana syllabary, work through the activities in this book. It will give you a solid foundation on two things - mastery of the kana (since everything is written in kana), and basic sentence patterns. If you still have the time, get a kanji book and start working on the first few hundred kanji. I highly recommend Basic Kanji Book. This way, you can correlate the kana spellings of the new vocabulary words with the actual kanji. However, your focus should be more on the correct kana spelling. I always get confused with the long vowel sounds.
About this book: real excellent introduction to Japanese vocabulary, culture, and grammar. The exercises can be repetitive and are mostly substitution exercises. However, this is a great way to master the sentence patterns. My suggestion is to do the exercises and then to create one or two other sentences of your own based on what you just did. This way, you can further apply what you have just learned.
Once you're done with this book, skip volumes II & III. Move on to either the Genki series or Minna no Nihongo series, although I'm somewhat partial to the Minna no Nihongo series because it's more comprehensive albeit more expensive. It covers reading, writing, listening, and composition. At the same time, keep chipping away at the 1945 Joyo Kanji. In time, you will have mastered enough Nihongo to either pass the JLPT or order your favorite sushi.
Follow Up: It's been about a year since I worked through this book. I take back what I said about skipping volumes II & III. I think it's complements well with either Genki or Minna no Nihongo in terms of supplementing your vocabulary with more current terms.