One of the best ways to improve your vocabulary in any language is to read more. I’ve never heard anyone disagree with that. It’s a highly recommended way for kids to improve their vocabulary, and the same goes for an adult learning a second language.
The challenge then comes in the form of the question: What should I be reading? Likely, many of the books you pick up in your language of choice early on feel completely overwhelming. There’s so many words you don’t recognize, so many grammar points that don’t click, and sometimes just strange cultural references that make no sense. It’s enough that after just a page or two, you feel exhausted and give up.
Alternatively, you might be picking up easy materials meant for children, or specially prepared readers for second language learners (like the excellent Mandarin Companion Graded Readers series). The problem with these is that they tend to be either boring for an intelligent adult or just too few available for you to be able to ramp up from beginner to fluent.
The trick then is to find material that’s interesting, but not too difficult as to be exhausting.
An obvious place that many people start with is reading translations of YA (Young Adult) books that they’re already familiar with in English. Harry Potter, Percy Jackson, and the like are good choices if you’ve already read them a million times, but don’t be surprised if they’re more difficult than you originally thought. They’re “easy” to you as an adult reader because their plot lines tend to not be terribly complex and they use a rather limited vocabulary, but only when you compare them with works like Atlas Shrugged, as a beginner in a language you still might feel overwhelmed, but you also might enjoy the plots enough to just power through the sections you don’t quite get anyways.
A less obvious place to start is non-fiction. While reading a technical book seems more complex than a YA book, in many cases if you’re already fairly familiar with the topic, you might find it a lot easier. For example, I recently started reading Los indoeuropeos y los orígenes de Europa by Francisco Villar, which walks through the history of our modern understanding of the people who spoke Proto-Indoeuropean, what their language was probably like, and how we know it. It’s a complex book, covering many different concepts from the fields of linguistics and archaeology and presenting academic, jargon heavy arguments about what various experts have thought over the years and what data lead them to their hypotheses. However, it hasn’t been an overly difficult book for me to follow in Spanish.
Because I spent several years studying linguistics, many of the terms and concepts make a lot of sense to me. Even if the vocabulary I’m used to isn’t the same as what the author’s using, the concepts are still familiar enough that I can fill in the blanks. Unlike with fiction, there’s only so many possible meanings a sentence can have and still make sense. Another benefit is that many of the verbs will tend to stay in the same tense and number throughout an entire section, which limits the amount of grammar processing my brain has to do at one time.
I’m not recommending that you go pick up a copy of this book, unless you happen to be into historical linguistics like me, but you likely are well familiar with some other topic that you could find interesting to read about. Studied computer programming in college? Pick up a “learn Java” book in French. Like dogs? Try to find a Japanese book about training your dog. Many non-fiction topics like this will have the same benefits as the Indo-European book did for me: Both the grammar and the vocab is guessable from context.
Aside from the topic of the book, I would also recommend that you read electronic books as much as possible in the beginning. Even if from context, you’re able to understand 95% of the vocab, you still might want to look up a few of the words you don’t quite get. This is made much easier with builtin dictionaries on Kindle or other e-reader. Many of these also can send highlighted text through a machine translation service, like Kindle does with Bing translator, which can help in cases where you really can’t make sense of an entire paragraph.
If you don’t have a dedicated e-reader, then you can always use your phone, tablet, or computer instead. Personally I find the Kindle well worth it as it encourages a less distracted reading environment and I find that the e-ink display is much better on my eyes for longer reading.
Remember: Reading in any language should be enjoyable and you should never feel obligated to finish every book you start. And the more practice reading things that are just on the edge of your comfort level that you give yourself, the better you’ll get.