If you're in the US or one of several countries that let you dike around for the first two years before declaring a major, I'd say to take some electives and see what you click the most in. I took a year of architecture at age 18-19 because it was what I wanted to do since I was about 9 or 10; then I realized I definitely didn't want to be an architect, so now at 22, I've begun again, having an idea of what I want to do (translation, technical writing [manuals and such], and teaching) and feeling around what exactly to major in during this first year (currently it's the nebulous-yet-available Foreign Languages major). So far I'm torn between Japanese or German, majoring in one language and minoring in the other; or choosing one of those languages (would like to have Japanese since it runs in the family and is in demand in some industries right now; a lot of German companies are moving to my state [Porsche and such] and we're itching for German translators) and then minoring in Linguistics, and yeah.
I'd say that one of the best ways to know what you want to do is get some exposure to it in school. When I was in architecture school, for example, I kept a close eye on my professors and the TAs whom I'd overhear talking about how there's absolutely no work in architecture right now, and from watching nearly every architect I met be a chain smoker or a severe nail biter. That spoke a lot to me how I didn't want to have a career that would cause me to take up smoking (especially to that amount) or begin disfiguring my body over the stress.
Which reminds me, you'll want to make a lot of lists of what you want out of a job (which you seem to have started) and what you enjoy, and how to get what you want by doing what you enjoy.
Also, this should work well anywhere, but it is America-centric, so again, if you're in the US, you may find thehttp://www.careerinfonet.org/Occupations/select_occupation.aspx?next=occ_rep&level=&optstatus=111111111&id=1&nodeid=2&soccode=&stfips=&jobfam= helpful. I have since high school. It shows you the average salary in your current or desired area as well as the average experience and education level, and the type of things required by their listed careers.
EDIT: I totally understand you on the worry about liking computers as a hobby. For a while, I wanted to be a network administrator but since I worked several PC repair jobs and am working one now as I go to school, I can tell you it burned/burns me out a lot mainly because I hate being the one to blame when things go wrong and it being out of my control. I also hate having to answer to angry people--e.g. customers and angry employees.