Intro: How to Become Fluent in Japanese Without Taking a Formal Class
Buy the book Japanese Grammar (Barron's Grammar Series) Carol and Nobuo Akiyama and memorize its contents. This book is a very valuable resource and covers most of the basics of Japanese Grammar. Here is a link .
While reading through the first book here is a list of resources that you should download.
Tagaini Jisho- This is very useful when first learning the kana and will remain a valuable resource as you learn the kanji.
Anki- A very good flashcard program. The flash cards that will be most helpful are
JWPce- This is a Japanese word processor and is also useful as a dictionary.
After learning the basics of Japanese Grammar from Barron you should learn the hiragana and at least familarize yourself with the katakana using Tagaini Jisho. Also use the Anki flashcards Master Hiragana (with audio) to familiarize yourself with the pronunciations more.
You should also try to expand your vocabulary using the Anki flahcards Genki I Vocab Master Deck. This also has audio so you can improve your pronunciation and listening comprehension.
You should also introduce yourself to some kanji using the Anki flashcards All Kanji for JLPT 4. Use Tagaini Jisho as a resource for breaking apart the kanji to manageable parts and for practicing stroke order.
After taking care to complete the first two steps completely, you should try using anime with Japanese and English subtitles. This can help to increase pretty much all aspects of your Japanese. You can find Japanese subtitles on the site kitsunekko.net. You can download pretty much any anime off of the internet so that should not be difficult. I suggest using Death Note, Clannad, or Lucky Star. They have a lot of dialouge and I know that the subtitles work for these. View these animes using KMPlayer. This player can change the subtitles to be viewed on the top of the screen. You can view both the English and Japanese subtitles if the English subtitles are hard subbed into the video and the Japanese subtitles are on top. If the subtitles are not hard subbed you can do it yourself using Any Video Converter. Just make sure that the settings for the conversion are Frame Size: Original, Audio Track: Japanese, and Subtitles: Japanese. After you have them hard subbed and are viewing the Japanese subtitles on top it should look like the picture on top.
Make sure not to get stuck on slang work with what you know and expand gradually from there. Most of the Japanese subtitles can be viewed as a text document so you can copy phrases or words from there. You can view Japanese Transcripts for some anime on http://yagokoroshi.wordpress.com/. Viewing this in Firefox using the add-on Rikaichan would be the most efficient way, I think. For grammar that you do not know use http://ejje.weblio.jp/.
For a final step. Increase your knowledge of grammar using Makino and Tsutsui's A Dictionary of Basic Japanese Grammar, A Dictionary of Intermediate Japanese Grammar, and A Dictionary of Advanced Japanese Grammar. After reading through the grammar points use the books index for quick review. Another valuable website that I forgot to mention is http://nihongo.j-talk.com/kanji/ this website has a very good dictionary and can convert Japanese with kanji into just the kana.
That should be a complete list of resources. I am not fluent in the language yet but I plan to use these resources to become fluent in the near future.