If I could only give one sentence of advice to a new student of Japanese, I would be tempted to say something like this: 
 

Continue learning new vocab every day for as long as it takes to become fluent.

I think that learning new vocab is arguably more important than studying grammar, pronunciation, speaking — just about anything else. This isn't to say that those other things aren't important, as they're pretty vital to our studies, as well. But vocab reigns supreme.

Why?

• Even if you know a ton of grammar patterns, if you don't know multiple words in a sentence, you won't understand it.
• Even if you can pronounce Japanese as well as a native speaker, if you don't know what words to use/say, it doesn't matter.
• Even if you can have basic conversations, if lacking an extensive vocabulary, it will be impossible to express complex ideas and understand much of what is being said to you.

Right. So, vocab is important. But how should we learn it?

We have a lot of options...