Once you identify schools/programs of interest, pay attention to admission requirements and deadlines. Many programs have application deadlines in the first quarter of the year for a September admission. Also, don't procrastinate in asking your professors for letters of recommendation. Give them time to write a nice letter!
Many programs require an standardized test for admission. Which test is required depends on the program and school. On this page, I've listed titles of some preparation books that may be of interest. The library also has a book for help writting Graduate Admission Essays.
These books are held by the Carney Library. You may decide to review these books before purchasing your own preparation manual. You can also use them as a supplement to your test preparation materials. If you're interested in purchasing a manual, publishers such as Barrons, Princeton Review and Peterson's sell preparation materials. There are many preparation manuals avialable on Amazon.