There are two approaches:
1. Proactive Approach
One way to figure out what you should learn is to try to predict what situations you’ll be in and what vocabulary you’ll want to use.
Anybody who starts to learn a new language must have followed this approach. Have you prepared for possible conversations in a supermarket? Being prepared for a situation definitely helps to open a dialogue and get the task done. Once used, the vocabulary, phrases or sentences are easy to be remembered.
However, there is still one drawback: the vocabulary learnt from the imaginary situation might be more than necessity; the unused words can be forgotten easily.
2 . Reactive Approach
The other way to increase your Chinese vocabulary is to learn the words you needed but didn’t have at the time when you wanted to use it.
Or, if your Chinese-speaking friends are patient enough to stop every time you don’t understand so that you can write down those new words and learn them later.
This is an effective way of learning a foreign language. The initial embarrassment of not knowing the words or phrases can be a real motivation for the learner to find out the meaning and thus memorize them.
Take my own experience for example, when I first arrived in Germany, I did not know any German. One day I wanted to buy some flour, but didn’t know the word. I went to the supermarket and asked the cashier, she did not understand me. I asked around, one lady seemed to understand and gave me the direction to a shop at the corner in a couple of blocks. I went there but found a FLOWER shop! The first thing I went home was to find out the German word for “flour”. And I will never forget the word “Mehl” any more.
Of course the better way of learning Chinese is to combine both approaches. On top of it, keep on practicing!