So you know your hiragana, a few hundred kanji and most of the basic Japanese grammar rules. What do you do next? In this article we'll look at some possible directions you can go in with your Japanese after getting a handle on the basics.
Fill in the blanks
At this stage your Japanese is good, but it's not that good. Could you read a newspaper without a dictionary? If so then you're probably close enough to native to not have to worry about studying Japanese anymore. If you can't, then this is what you're aiming for: to be able to read Japanese language media for natives.
To get to that stage, the vast majority of the work will be learning new vocab. Doing this from textbooks can be quite boring and relatively slow, but doing this from material for natives (newspapers, books etc) can be quite a lot of effort to start with.
Focus on your interests
If you're interested in gardening, then learn gardening vocabulary. If you're a professional programmer, then learn all the Japanese equivalents of technical terms. Focusing on words that you would use a lot in a Japanese context makes it a lot more personal to you.
This is not to say you shouldn't learn vocab outside your interests. Your goal is to be a literate, adult native in Japanese, so you need a good working vocabulary.
The benefit of starting from your interests however is that no matter what you're reading, most of the vocabulary is going to be the same. If you're watching Star Trek in Japanese, 99% of the vocabulary will be regular Japanese words.
In this way you can sneak learning a fully-functional Japanese vocabulary into reading things that interest you.
According to Stephen Krashens input hypothesis, the best way to learn a new language is to be exposed to as much comprehensible input as possible and books the target language are the best source.
I'm finding that the best way to get started with Japanese books is to find translations of books you've already read in English. This way you're not totally lost if there is a lot of vocab you don't know.
If you have an SRS system, it pays to enter any new vocab you come accross. That way you can still review the vocab even if your schedule doesn't allow you to focus on reading Japanese regularly.
Use the internet in Japanese
A little less hardcore than books, but using the internet in Japanese regularly can help you get used to the written language. Browsing the Yomiuri online in Japanese or following people who tweet in Japanese can be a good step towards this. Coupled with browser plugins like rikai-(chan|kun) it becomes really easy to consume Japanese content.
This is an even better source of new vocab than books, because it's common, colloquial and up to date. It's the Japanese that people are using right now. It also allows you to see the world (or the internet at least) from a Japanese perspective, going some ways towards plugging you into Japanese culture (not the 'ten wacky gameshows from Japan' point of view that you might have as a Gaijin).
This to me is as important as learning grammar or vocabulary. You may understand the words Japanese people say perfectly, but without cultural context much of the meaning is lost. Humour is the litmus test for this, if you can watch comedy shows and "get" most of the jokes, you're on your way.
Closing Note: Keep Going
Learning the basics of Japanese really is the easy part. The long hard struggle is going from that basic knowledge to a fully-functional grasp of the language at a native level.
Intellectually it isn't very challenging, but it takes a long time to expose yourself to enough examples of the language to become truly fluent. Early, often and in small doses to start with, then build from there.