Take a GMAT practice test. You can find practice tests on the GMAT website, linked in the Resources section.
Review your test, making note of questions you did not answer correctly. Separate them by type, concentrating on any areas where you got more than question on the same topic wrong.
Review the topics you felt you did not understand in a math textbook. You can use a textbook specific to the GMAT, or any math text that covers the same concepts.
Practice solving math problems using these concepts. Assign yourself some time each day to review principles and practice applying them to problems.
Take another practice test. Check the questions you didn't get right and continue to review those areas. Continual practice will help build confidence and familiarity with the GMAT math section.