You have PLENTY of time to work this out.
When I was at school, all I wanted to be was an Illustrator for children's books. So I concentrated on my Art A level and little else. I left school and spent a year at art college and then decided to get a job and the first that came up was in a shop job. I didnt realise my dream until many many years after leaving school.
Since leaving school I have worked in many different industries, all of them really interesting and fun: pest control, web design, administration, retail, hotel, MOD, technician, Civil engineer, art gallery steward, photographer. At the age of 30, I settled in engineering, liked it and my employer paid for me to get qualified in civil engineering. I thoight this would be my career choice until I retired, but it wasnt.
Now I am nearly 50 and I have just left the engineering industry to do what I have always wanted to do since I was at school and that is to be a Professional Illustrator. So I have come full circle.
My son has just left school and he has no idea what he wants to do but that is absolutely fine, because it will come to him one day. There is no rush. I didnt get very good grades for going on to college afterwards, when I was at school, but I made up for it after I left school, with work and experience and my employer believed in me.
Dont worry, enjoy life. Be dedicated to whatever it is that you choose.Try as many interests as you want, or stick to one, you will be fine.