As you do preparation for the IELTS, PTE or TOFEL exam test, you work through classes and materials. During preparation for the exam, you will increase your vocabulary in the process. Having and knowing more vocabulary will help you read faster and understand the text more easily. Keep a notebook and add any useful new words with word meaning, type (noun, adjective, adverb), pronunciation, expressions, a couple of sentences, synonyms, antonyms and/or collocations to it, so that you can see how the word or paraphrase is used in English.
One very important focus is on expanding the range of your academic vocabulary and developing your awareness of paraphrasing, which means different ways of saying the same thing to tackle any question at the test, especially in the reading module. Exam questions will rarely the same grammar and keywords as occur in the text.