Studying the manual available from the Department Motor Vehicles, taking practice tests and talking to people who have passed the test recently are the best ways to prepare for a DMV permit test. The learner's permit gives you permission to start driving with a licensed operator.
While most of the signs that appear on U.S. roads are uniform, some of the rules of the road vary from one state to another, so obtain the manual specific to your state. These are available from your local DMV branch. Read each section, and take the quiz at the end of the booklet, if your state's manual includes one. The test involves understanding the road rules specific to your state.
Websites such as Driving-Tests.org offer free practice tests to let potential test-takers determine whether their studying has prepared them for testing. Online tests often provide immediate feedback, allowing you to fill any gaps in your knowledge. Because of the small number of questions on many state permit tests, you can often fail after missing only a handful of questions. Talking to people who have taken the test recently gives you perspective on what the testing environment is like and an idea of the questions that appeared on their tests.