This is a million dollar question !
Some people are very lucky , those who realise what they love doing pretty early on. They have encouraging parents who allow them to pursue their dreams.
But then there are the other bunch like me who wander aimlessly unable to figure out what they like. We either like everything or like nothing. so what do you do when one is like that.
Start thinking about your broad interests. Do you like sports, do you love designing things, you might like travelling or even love working on math problems
Draw a list of top few things you love doing and then try to filter down to the career options in that area. If you are a good writer and love sports, think about sports journalism. If you like maths think about pure math research
Next step is to figure out the basic degree needed for your interests . Since Sports/Journalism are not too structured you need build portfolios to show your capability. Start building it by blogging. For pure research you need to be very good in academics and score good marks.
I hope you find this information helpful see you on the next topic.