Actually, most people today have more than one career. You may end up with three or four careers made up of ten or more jobs. So none of the choices you make now are forever or irrevocable, but they are important. So how do you decide on a direction?
Take a look at yourself. What do you enjoy doing, what are you good at? Hobbies, activities and coursework can all give you insight into the skills you would enjoy using on the job. Also consider style. Are you a laid back sort of person, or buttoned-up? Office oriented or outdoorsy? Different work environments appeal to different people. Consider your values. If you need some help, Career Services has several "tests" which can help you identify career-related interests.
Investigate what people like you like to do. You can read books and articles about jobs for people who love children, or the outdoors, etc. Career Services has some to lend, as will your library. Talk with faculty or other people who share your interests, and ask for their suggestions. "Testing" can also be useful at this stage.
A favorite technique is to read the employment classifieds A through Z for six weeks straight, cutting out any job that appeals to you for any reason at all. After you've finished gathering your "data", analyze it for trends. Do you most often cut out financial jobs, or jobs working with kids, or jobs in zoos? People tend to focus either on what they like to do or where to do it. Either is a good starting point. Try to find 20 jobs that give you a tingle. You should have a good idea, and a lot of leads for later, when you finish this exercise.
Research the top runners. When you have a number of options, do some research. There may be something you'd love to do, but which takes 8 years of schooling, and something else you would enjoy which takes 4. If you like school but don't love it, perhaps the latter is the wiser choice. Find out the qualifications, the potential salary- important information like that.
Do Information Interviews. Talk to people who work in the field in which you are interested. Make sure the job and environment is really as you imagined.
Make a Choice! This is sometimes the hard part, but you need a goal if you're going to get moving.
Get ready. Most careers will require specific education and experience. If you don't already have what you need, make a plan to get it. Internships, summer jobs and volunteer work all count.