Each branch of military service has their own standards of enlistment and retention, so you should refer to the branch that you are interested in to understand their specific requirements.
Generally, like any other job, to turn it into a career, your performance has to be good at the specific job that you have chosen. So, it is very important to choose a career field that you think that you might enjoy because a career is a lot of working years.
In addition to job performance, all military service jobs require a certain level of physical fitness; again this varies by service branch. Each branch has their own policies about criminal activity, moral behavior, and drug use, but within each branch there are even more strict guidelines based on career field dictated by security clearance levels.
Lance recommends the following next steps: