You walk in to the recruiters office and tell them you would like to join the army. The recruiter has you fill out a bio sheet and creates asks why you're interested and what you're interested in doing. They explain some of the benefits you can receive and will start to look up jobs for you to do. Not everything is available. If the job you want is not available, you can wait and one day, if you're lucky, there will be a spot for you. Otherwise you have to decide if you want to accept something else, if you wait to long to decide, then that job will also disappear to someone in another recruiting office somewhere else in the country.