You must possess a High School Diploma (Some College or Associate's degree preferred) and you must be able to take and pass an aptitude test (ASVAB), drug test and a physical exam. For most jobs you will need a security clearance which will require a background check and a credit check. You must speak English (as a first or second language). The Army may choose to pay you extra if you are bilingual or trilingual in a needed language such as Arabic or Russian.
You will need to contact a recruiter, that recruiter will assist you with your application and will present you with options to be promoted before you leave for Basic Combat Training. Your recruiter can help you answer any other questions.
Pro Tips:
Speaking to a recruiter does not imply an obligation to join the military. You can stop the process at any time before you sign your contract at a Military Entry and Processing Station (MEPS).
Ask your recruiter for a technical job such as Network Operator or Air Traffic Controller. You'll thank me later when you're making 80,000 - 100,000 in the civilian world after you get out of the Army.