Determine the state in which you wish to teach. Every state requires specific tests and particular scores on that tests. It is common for many states to require the nationally recognized Praxis I (and sometimes Praxis II) in addition to a state specific test. Research the test type and score requirements for the particular position you want.
Purchase test preparation guides to prepare for the test. These can be purchased at almost any major bookseller. The Praxis I and II exam guides are available online and can be purchased used for a fraction of the retail cost. These books offer practice tests, so aspiring teachers can see the types of questions that will be asked before taking the exam(s).
Review academic information and content knowledge. The Praxis I tests the basic education level and the ability to teach basic K-12 skills and information. The Praxis II is primarily for secondary educators and tests specific content knowledge. In all exams, not just the Praxis series, you will be tested on your ability to recall specific information from the content area you'll be teaching.
Check registration deadlines. Register to take the exam in the late fall or early spring one full year before you begin teaching. Though some state boards of education will grant a provisional (temporary) license while testing and education requirements are completed, these can often run out in one or two years. If hired on a provisional license, you will be fired if you do not successfully complete the exams by the deadlines set by the state board of education. Give yourself plenty of time to take the exam and re-take it if you don't pass the first time.
Get copies of your exam scores to provide to the state board of education. They will need official copies before you'll be granted a teaching license.