I am not a professional.
What ever you think you need to learn Japanese, put it to the back of your mind right now. If you're willing to take an alternative point-of-view, I'll tell you the secret (my secret) to learning Japanese or any language.
You'll need tools.
Hit the highlights or journey with me through a metaphor. Think of Japanese (or any language) as a house. Learning the language is comparable to building that house. The roof and exterior are your communication with the outside world, the paint and styling of your interiors represent your special syntax or dialect (unique to each person in my opinion), and all the boards and bricks between the inside and the outside can be compared to every word in your vocabulary, making the language complete and structurally sound even though you don't know every single word in your native tongue. The house is big and unique to each of us in our native tongue.
And it starts with the foundation.
Grammar is the foundation of language. Any language. No one bothers thinking about the foundation of their native "language house" because the structure is completely in tact at a very early age. We spend time studying grammar in school, yet without even bothering we can produce long, complicated thoughts in coherent sentences - whether or not our grammar is perfect (take my punctuation as an example). In fact, when you move into a house in real life you don't bother thinking about the foundation that much. The house is built and livable. Just like your native language. However, contract a company to build your "dream house" and suddenly you are very much involved with the process of laying a foundation. Anyone who has been involved with building, or contracting someone to build, their own house knows that a foundation is not just a concrete slab. Everything you need to make the house "livable" has to be planned out in the foundation. Water is the most essential element. Without running water most of us wouldn't bother moving in to the house. Water is vital to make a house "livable." Grammar is vital to make a language understandable.
A long metaphor - but grammar is to language what water is to life. And modifying a very famous saying, "grammar is like water, it isn't important until you aren't getting any."
So rule number 1 is: forget about English grammar.
Start out with a fresh spot in your mind somewhere near your sylvain fissure, wipe out any remaining English grammar, and prepare to lay a new grammatical foundation for the Japanese language. There are many pieces of the foundation to lay out, but first...
You'll need tools.
I study Japanese everyday armed with the tools of the trade: dictionary, flashcards, Japanese chat sites (I tend towards IRC, but there are plenty of Japanese chat sites), and a professor's office full of books about Japanese.
So before you dive in to putting together a grammar foundation for Japanese, arm yourself with the proper tools:
Dictionary: Japanese-English Dictionary Interface (JEDI). This is an extensive Japanese-English dictionary that handles input of English, hiragana, katakana, and complex kanji. The dictionary will not return results for single kanji, or single kana (if you were using the on-yomi to search for a kanji).
Flashcards: Japanese-kanji.com. This site has an amazing Java applet for quizzing yourself on all 1945 Joyo Kanji in either their traditionally listed order or the JLPT order. This is the best site for kanji learning/recognition practice that I have ever found.
IRC: There are lots of channels on IRC frequented by Japanese people or Japanese speakers. Sometimes the best way to learn another language is from someone who speaks your native language as this person has encountered lots of the same problems with grammar and translations you will likely encounter during your studies. I am a staple on EFnet #japan and #nihongo
Books: There are, at last count, 2039570472935 books about the Japanese language available on 43059673945 different internet retailers. While I haven't used them all, I highly recommend the Genki series created by former students/faculty members at Kansai Gaidai University in Hirakata, Japan. There are many others, but these are great books for those just starting out through intermediate. Also, just because you are not a child doesn't mean you should forget the simple lessons you learned as a child. Most of these lessons came from fairy tales or other children's stories. So read them in Japanese. I recommend ももたろ (momotaro - English version). Find other stories in Japanese at your discretion. The best part is that children's stories in Japanese are mostly written in kana with very few kanji (with side-by-side kana readings). READ!
A final piece of advice for anyone starting out: forget anime/manga.
While not all people interested in learning Japanese have heard of manga, the vast majority of foreigners learning Japanese have either heard of manga/anime or are dedicated fans (or somewhere in the middle). I try not to impose any of my personal biases when teaching, but anime/manga is ruining the average Japanese student's impression of the Japanese language.
I'll explain -
Anime/manga are Japanese cartoons, either in serialized-book form or video (TV, DVD, VHS, whatever). Just like American cartoons, Japanese cartoon characters are famous for the quirks that make them unique. Often these quirks express themselves in language, and characters are renowned for their catch-phrases or special dialect (accent, slang, etc.). Everyone in America knows (or should know) The Simpsons. Everyone I know uses and understands the expression, "doh!" Without the world's most famous cartoon dad, Homer Simpson, that phrase might not ever have become common use in the English language. This is the danger of picking up words from cartoons!!! Using "doh!" in common speech and studying "doh!" as common speech are two separate issues.
Native speakers know intuitively where the word "doh!" comes from (or can at least recognize it is not a word learned from a class in school), but a foreign student studying English might not. The student might come to think that everyone who speaks English uses "doh!" as a natural part of their vocabulary, a formal part even. This is exactly what I hear from Japanese language students who spend a lot of time watching/reading anime/manga.
The best example I can give is the term ～でござる(degozaru) made famous by Rurouni Kenshin, a samurai character with a cult-following. Degozaru takes the place of ～でございます (degozaimasu) - a formal ending for polite speech in Japanese. While there is no exact translation in the English language (a word or phrase we use in exactly the same way), you could probably think of ～でございます(degozaimasu) as something like "if it pleases you" the way you might hear it in very formal, and "oldy" English, conversation. ～でござる(degozaru) follows a sentence that can end in です(desu) or だ(da). This expression was used mostly by Samurai during the Edo Period. For that reason, it is a highly recognizable phrase, something all Japanese people will understand. But using it as a normal part of conversational Japanese is as funny as saying "doh!" during a congressional meeting. Some people may laugh, others may look at you as though you are a madman. I have seen both.
Anime/manga language is wonderful and fun precisely because it is anachronistic in common speech. If you are a native English speaker, imagine an American or Australian with a thick accent using Elizabethan English with their best impersonation of a period accent (though not entirely accurate). On stage from a comedian, this might sound really funny. At the supermarket when you are buying food, this would sound really strange.
Always keep in mind that your pronunciation and mastery of the Japanese language has to be 10 times better than native Japanese speakers because Japanese people have a hard enough time believing Westerners can speak Japanese in the first place. When you use non-standard phrases like ～でござる(degozaru), it serves to totally confuse a native Japanese speaker. They can't be sure if you are intentionally being funny or not.