The standard academic recommendation source for future career evaluation is called Occupational Outlook Handbook. This well used and oft recommended source, while it has a broad range of information about a vast number of career opportunities (and is easier to use now that it is available over the Internet) may not serve the purpose of those who need a more personal, as opposed to statistical, approach to discovering career aptitude and possibilities. In other words, the detailed list of statistics and definitions may leave one more confused than directed.
Another standard academically recommended (though ofttimes overlooked in by academic counselors) source for future career evaluation are aptitude tests and personality tests (i.e., pay attention to the results of your school aptitude tests where/when given). Aptitude tests, which are of varying kinds, can accurately point you in correct directions for future careers that accord with your natural aptitudes. Personality tests can provide the same direction and, when taken in consort with an aptitude test (be sure you use test that are academically recommended), you can get valuable confirmation of results and career directions.
In addition, though subject to a less favorable opinion these days, IQ tests can be useful in helping you to find general directions. For example, if you score 5 out of 100 on spatial orientation, you will know that mechanical and engineering work will not be the best suited for you as you have no ability to manipulate spatial relationships. Similarly, if you score 47 out of 100 on mathematical calculations, you will know that work requiring mathematical excellence will not be suitable. In contrast, if you score 99 out of 100 on language, you will know that careers dealing with words and abstract concepts will be suitable. Not all tests will give such cut-and-dried results as my examples, but the principle applies that IQ tests can help you decide appropriate directions in career choices.
Many universities now provide online information about careers and educational pathways to those careers. For example, the University of Manitoba offers useful links to detailed information about a wide range of career possibilities while the Florida Department of Education offers a "career cruiser" that provides aptitude tests and career information.
When it comes to figuring out what you want to ultimately do with your life, it is a little disheartening, and somewhat ironic, that, in America anyway, you must make major educational decisions with little-to-no life experience, and at an arguably young age. This is why so many people change majors in college, or end up going back to school later in life for a completely different career interest.
That said, I think there are a few things you can do, even as young as high school, as research for potential career options. First, look at your interests and identify your strengths, academically and otherwise. It is becoming more and more common in four-year universities, to have incoming freshman take personality tests and surveys to help them narrow down career fields based on their interests, experience, and personality strengths. One great book is Strengths Finder by Tom Rath. If nothing else, you can certainly make a list of areas to completely avoid based on things you know you cannot enjoy or do.
Based on your lists of interests and strengths, the next logical step is to come up with a list of career options that sound like something you could do. Many online surveys and tests will create this list for you but you can add to it from what you already know. Other available resources to use are teachers, career counselors at school, your parents, and other adults you have personal connections with.
Once you are ready to really narrow down your field (whether that be in an academic persuit, or in persuit of a trade), based on experience alone, I now advise all of my students to actually meet with someone and talk, face-to-face, about the ins and outs of different jobs, companies, industries, and career movement. I also sincerely encourage students to consider the lifestyle they envision themselves living one day, and to calculate on a basic level, if the career they are choosing will allow them to afford their desired lifestyle.
Unfortunately, our society is currently suffering from an economic and financial "crisis" and those who are the most affected, and the most left-out of the information circle, are the college aged and the recent graduates who have high ideals and expectations, possibly a significant amount of debt, but very little knowledge of "the real world."
Good luck in your pursuit to discover a career that will fulfil you emotionally, intellectually, and financially. They say the key to success is doing what you love, and those who can make a life built on this idea are lucky indeed.