Improving your vocabulary in order to do well on the ACT is simple in some ways, and difficult in others. You can get any number of books on increasing your vocabulary designed for those taking the ACT and/or SAT exams. These books are designed to familiarize you with the typical words that you'll find on the tests. They will give you synonyms and proper use in a variety of sentences. They may give you alternate meanings and usages and possibly word origin - all designed to help you do well on the test. They will offer mastery quizzes to reinforce your learning. This is the simplest way improve your vocabulary score on the ACT and/or SAT.
The more difficult way to improve your vocabulary is to start very early in life being an avid reader. Reading books will boost your vocabulary, but it takes a lot of time and effort. Unlike cramming for the test with vocabulary building books, this will be a more certain way to improve your score. You will retain the words better, and you will incorporate these words into your speaking and writing, which will increase your reading comprehension as well (which will improve your overall language score on the tests, as well as your overall academic performance.