The easiest way to join the United States Army is to visit a recruiter at an Army recruiting office. There is also an online application form, but it is only for pre-qualification and talking to a recruiter is still required.
You have to meet certain basic requirements to join the Army. You must be a U.S. citizen or legal resident and be between the ages of 17 and 34. You must meet certain health requirements and have a high school diploma as well. The recruiting process may start before you complete your high school education, but you cannot actually join until you have graduated. There may be waivers available for some requirements, which a recruiter can help you determine.
Army recruits also have to take the Armed Services Vocational Aptitude Battery test, commonly called the ASVAB, which measures potential for various jobs. Along with personal preferences, it is the test that largely determines what job you are assigned to when you join. It measures knowledge in some areas and logic in others, but it does not require any special studying or research.
Many of these tests take place at a Military Entrance Processing Station. Visiting an MEPS is the final stage of the enlistment process and it is usually where your ASVAB is administered and your physical is completed. If you meet those requirements, the next step is fingerprinting and a background check, then processing the enlistment paperwork.