i know that a job is right for me if i enjoy it! of course, if you cant immediately get your dream job (you have to go to school, it takes a lot of work to actually start earning money [like youtube or twitch] or you just can't start right now) then you'll probably have to work a minimum-wage job until you can start your dream job
I know i wanted to be an actress because one day i just decided to try acting to see what it was like, and when i started I knew that acting was what i wanted to do with my life, because i loved doing it, so if you don't know, then just go for something you enjoy! (if you're interested in medicine, maybe consider becoming something related to medicine for example)
But even if you still don't know after that, i'd say just hop around for a little while until you find something you're content with