Finding the vocabulary you should learn is the easy part, actually learning it is much more difficult. There are two main ways vocabulary can be learnt and there are a few terms for each way; intensive/ explicit learning is when you purposefully study words through lists or word cards etc. Extensive or implicit learning is when you come across vocabulary in daily use, such as through reading books or talking with someone who doesn't have to be a native speaker.

Because extensive learning of vocabulary is for the most part done through context, this way is thought to be the most effective, however it is much more time consuming. They say that to be able to learn through context, at least 95% of words in a text need to be known. This leads me to the lexical paradox as highlighted by Tom Cobb; in order to be able to learn vocabulary extensively, we need vocabulary. This means that during the beginning stages, intensive vocabulary study is paramount, and as the number of vocabulary acquired increases, learning through context becomes much easier. 

If you want a quick way to vastly improve your vocabulary, do a lot of extensive reading in the target language, but make sure that you do it at the right level; more than 95% of words known. In order to become a quicker reader, this number needs to be higher than 98%.