This is from the airforce.com Q&A..

Enlistment into the U.S. Air Force, or any branch of the U.S. military, by citizens of countries other than the United States is limited to those foreign nationals who are legally residing in the United States and possess an Immigrations and Naturalization Service Alien Registration Card (INS Form I-151/551 -- Commonly known as a "Green Card"). Applicants must be between 17 and 35; meet the mental, moral, and physical standards for enlistment; and must speak, read and write English fluently.
Note: The U.S. military branches cannot assist foreign nationals in obtaining admittance into the United States. Questions concerning immigration to the United States should be asked of the U.S. Embassy. Only after immigration procedures are completed and an applicant is legally residing in the United States may an application for enlistment be accepted.
Furthermore, in order to be commissioned an officer in the U.S. Air Force, one must be a native-born or naturalized United States citizen.