Is there any way to know what one really wants to do? I mean, how can I be sure about liking a thing/career/person/life?

I am utterly confused, please help me with this.
Okay, you only help me understand about “My Personal Career Choice” at the moment, I shall ask about things, activities, persons, lifestyles etc. later.
I am finding it hard to know what career is it that I should be into, I mean, I have only a very vague idea about my liking, is there any website out there which can help me find the career I should belong to and why so?
Thanks a lot.