How can you know which words are the most important in a language? One way linguists find this out is by taking large samples of language and storing that in a computer database. They can run tests and analyses on the data, such as finding the most common and least common words in a language. This is called corpus linguistics.
More recently, some researchers have looked at written and spoken academic language to find the most common words used at the college level. This list is called the Academic Word List, or AWL. If you are preparing for college in the U.S. or some type of college entrance test (including the TOEFL), these words are essential to learn.
There are many websites that use the AWL and provide exercises to learn these words. Here are just a couple examples that you may find helpful: