I recently started using Tae Kim’s Complete Guide to Japanese, which helps even people with no knowledge of Japanese whatsoever get started on learning the language. Though learning to read and write Hiragana and Katakana take some time, it’s well worth it, because Tae Kim’s guide concisely and clearly explains every mechanic of Japanese grammar in conjunction with vocabulary that you would definitely find useful. For all you that fear Kanji like the plague, Tae Kim backs up the argument for the use of Kanji with sound reasoning, and also doesn’t dump twenty Kanji on you every chapter, which is helpful (though I myself would prefer that the font be a little bigger; my eyesight is terrible as it is).
For those you that are like me, preferring to learn a language’s grammar completely, and building vocabulary afterward, he also has a Japanese Grammar Guide, which is equally useful. I personally use a mix of both guides, especially for clarification on more complicated topics.
For all prospective learners of Japanese, I would not consider it an overstatement to call Tae Kim’s Guides to Japanese the definitive Bible of learning Japanese. ありがとうございます、キム-テイサン!