Take a practice test to determine where you need to focus your study. The ACT website has an online test that will give you immediate feedback. There are also numerous ACT study guides available in bookstores or online that contain practice tests.
Create flashcards for the math questions you had incorrect. On one side of an index card, write the problem. On the reverse side, write the answer including the steps to produce the answer.
Take 10 minutes every day to review one or two math questions. Spend more time if the test is only two weeks away.
Create vocabulary cards for the other three subject areas. Spend at least 10 minutes per day learning and reviewing vocabulary. Have friends or family quiz you while waiting for the bus, traveling to and from school, etc.