You can choose your career for at least three to four years. The military test is called the ASVAB test which lets you find out what job you are responsible for. If you choose a field to work with, you must have a high score in ASVAB before you can enroll in that field.
Head to the military's ASVAB page to take the test and see how you do it. You can also view the minimum ASVAB required for each job and see if you qualify. Visit Andy’s specialized online ASVAB course for testing tips page to learn how you can prepare for the test for the best results.
While you are doing this, take the time to research the various jobs in the military branch and find the job you think is the best fit for you. You should look at jobs that will be successful after you retire from the military, as well as jobs that you find suitable.
One thing you should know about joining a particular office is that all the recruiter hires who talk to other offices are trash just to get you to join theirs. Ignore it and choose a branch that can give you what you want.
Visit the Air Force, Army, Marine, and Navy websites and see what career fields they have. There are several good tests available on this website that categorize your interests into different categories. Then your answers will be aggregated to provide you with a list of jobs where you have shown interest.