Except in rare cases, no one can become a naturalized U.S. citizen without first getting a green card. To qualify for citizenship, you must have held the green card and been physically present in the U.S. for a certain period of time. You should also be a person of good moral character, have a knowledge of the English language and be familiar with American government and history.
In addition, you must also be at least 18 years old. Most people, in order to qualify for naturalization, are required to have held a green card a minimum of five years. At least one half of that time must have been spent physically inside U.S. boundaries. If you leave the country and remain absent for a year or more, this wipes out any time counted toward the five year total. Absences of fewer than six months do not affect the five year waiting period. If you are married to a U.S. citizen, you need have only held a green card for three years to qualify for naturalization. You may file a naturalization application no more than three months before any required period of U.S. residency has been completed. If your American citizen spouse is employed abroad by the U.S. government, there is no waiting period and no required residency period to get U.S. citizenship. You may apply for naturalization as soon as you receive your green card.