every language consists of 4 main pointers that a learner should master: reading, speaking, listening and writing. To succeed in these pointers, you should have a good vocabulary (kanji and kana) and grammar knowledge. There are a few good detailed websites out there dealing with vocabulary and grammar:
As for reading, you will first have to master hiragana and katakana atleast to be able to read books for children or manga which has furigana (these are little kana written on top of kanji, to my knowledge all manga have these). I don't know if there are any websites that could help you on your reading, and buying japanese books on ebay may be expensive. In Japan manga books are very cheap though.
As for speaking, if you don't have a japanese friend it will be hard for you to practise. Try to learn the pronounciation first (which is pretty easy) and try to speak yourself and maybe record it to listen to yourself.
As for listening, I can advice you to watch some japanese drama's online. They are far more better for you to listen to Japanese than any anime out there or music. Go to
Japanese Drama Home :: jdorama.com. They have a great database of Japanese drama's. Just enter a title of one of the more popular drama's in youtube or dailymotion (
www.dailymotion.com) and you will definetely be able to watch some complete drama's. I can recommend some very good drama's to you if you are interested..
And finally for writing, you can easily practise this yourself. First practise how to write the hiragana and katakana symbols (you find tons of information on how to write hiragana and katakana on the internet, remember that google is your best friend.). As for kanji, you will also find enough websites that have all (useful) kanji but it may be harder to find a site that also deals with the correct writing. This is one site I found which is perfect for you to study the writing but it only has 300 kanji (
http://www.dartmouth.edu/~kanji/index.html). Anyway, maybe you shouldn't worry about the writing part too much cause with computer nowadays you don't need to have any writing skills. Of course recognizing kanji is still vital.
So my opinion is; internet offers tons of information for you to start with. It would be a shame for you to buy books and maybe in a few months you realize that Japanese isn't your thing. I personally have always used websites for my studying and im really happy with the improvement I have made so far.
Best of luck !