Contact your university's office of the registrar. Ask when your college placement tests are and if it can suggest any materials to study from.
Give yourself about a month of time and preparation to study for placement tests. It will take time to get the materials, and you do not want to cram a lot of information at once because it will not do you any good.
Order the material that your university suggests to study from. If it does not offer any options, go to Google.com and enter "College Placement Tests." There are many free sites that offer practice tests for math, reading and writing. Another good resource is "College Placement Testing for Dummies."
Set aside three days a week to study for approximately an hour. Look through the books and focus on things that you are weak in from each subject. Concentrate on one subject a night.
Allot the three nights before your tests to take practice tests. The first two nights take a practice test for each subject; then the third night concentrate on the sections where you did not excel.