Tae Kim’s Japanese guide to Japanese grammar
This is the best site I’ve found to learn Japanese from. It teaches all the kana, then starts introducing the different particles used. Unlike most Japanese sites, it uses hardly any romaji, which is basically japanese words written in english. This is good, as in Japan nobody writes in romaji, so it’s a bad habit to get too used to it. There are plenty of examples to learn from also, and for the first few lessons there are also practice exercises.
The Kanji SITE
This site covers all of the kanji found in the first three levels of the JLPT, the Japanese Language Proficiency Test. Although this won’t teach you much about the way Japanese is used such as grammar and spoken conventions, it does teach a significant number of the 2000 or so kanji which are considered to be commonly used in most written Japanese. I know about 50, but it’s a good place to start.