Nearly every day, I see posts up in /r/LearnJapanese and similar forums for language learners, asking what kinds of resources are good and how to learn from materials made for native speakers. These people are definitely on the right track. You need to be doing a lot of listening and a lot of reading if you want to make progress in a language. Books, comics, video games, movies, blogs, magazines, web pages–grab anything that interests you. You’ll get a lot more out of it, though, if you have a solid strategy for using the materials.
First off, how do you know if something is good for your level? Graded readers are pretty good for this purpose. A graded reader is a book written with learners in mind. Usually, they have simpler grammar, a limited selection of vocabulary, and a fairly straightforward story that won’t add confusion. Many times they’ll have levels organized around a popular test, like the JLPT for Japanese or TOEIC for English, and those are okay as rough approximations. For beginners, these may be the only really good resource available, at least until you’ve built up a solid base of core vocabulary.
Children’s books are actually not that great for beginners, since they use a lot of colloquial words and onomatopoeia, which are way harder for adult learners to acquire. For lower-level learners of Japanese, you don’t really need to learn words like 裃(kamishimo, a samurai outfit) or 臼 (usu, a mortar for pounding rice flour into mochi), and yet those are exactly the sorts of words that fill Japanese children’s stories. It may seem counter-intuitive, but fairy tales are actually a much better resource for advanced students trying to build up some of the less common, cultural vocabulary. Think about examples from English fairy tales. How useful is the word “riding hood” for a first-year English student? Does an intermediate learner need to learn the expression, “all the better to…”? If your goal is to study abroad at an American university, you don’t need to learn what a “woodsman” is.
The best measure is to read a couple of pages and write down any new words or phrases you come across. 95% should be your cut-off line. That means that for every twenty content words, you’ll only have one new word in the bunch–that comes out to about one per paragraph most of the time. At that rate, you can usually guess the meaning from context, so you don’t have to interrupt the flow of reading and look things up just to understand the message. Easier texts are fine, though, because you’ll still be getting valuable practice decoding your target language and using the language structure to gather meaning. This kind of reading is called extensive reading, going for volume and enjoyment. It’s very different from what your brain does in intensive reading, which is slow and careful, used for trying to puzzle out the meaning of very difficult texts.
So now that you have some materials that are appropriate to your level, how do you use them? You could just read/listen/play straight through, and that would be fine for practice. If you want to get a little more out of it, though, and build your vocabulary as quick as possible, keep a notepad next to you.Write down new words that you find, and look them up later. That last part bears repeating: don’t look up vocabulary while you’re enjoying material! Play the game, read the comic, watch the movie, and just focus on doing that. Try to let the language fade into the background if you can. When you notice a word, write it down, but don’t let it disrupt the flow of study. Then, maybe every hour or so, or in a separate study session, look up the words.
And don’t just stop at looking up the words! Make flashcards while you’re at it, and practice them. You are using Anki, aren’t you? I recommend adding about 10 new words to your vocab deck every day. If you spend 30 minutes to an hour reading, watching, playing or listening to appropriately challenging material, you’ll have no problem finding those 10 words for the first few years of study. What’s more, you’ll be adding the vocabulary that’s most relevant to you, because it came up in material that you chose and that you enjoy.