Eligibility requirements highly depend on what state you live in. Typically, you must be a legal resident of the state, 18 years of age (17 in some states), and have been out of school for at least 2 months to 1 year.
All United States citizens are eligible to take a GED test. You only have to register to take it and pay the fee. If you are under 16 or 18, your state may require you to reach that age before you can take the test.