You can only discover by trying, living, even if you are gonna fail sometimes, that is how you realize that this is just about leaving the comfort or fear zone to discover the new and learn by itself
Try to know few things about every thing then ask yourself which field I interest or be happy when know information about it
This is a very philosophical question and I'm not a philosophy major. The closest thing I could say to an answer is be observant. You'll start to notice things and if you're interested in learning about them, you'll have the opportunity. Sometimes you don't know what life has to offer until life shows you. Or another person could show you, one or the other.
Strat by learning little of everything, very little, then something will catch you the most. Make it a priority to keep learning about it daily