Hey! First off, shush your little head when you say "I know this isn’t a serious problem…" it's bothering you, so you go right on ahead.
Right, I used to be exactly like you. Didn't know what on earth I wanted to do and I've only just found out now I'm 20.
The first thing you ask yourself is this:
If money was no object, what would you do?
For example, lets say that you say horse riding. You would love to spend your life on a beach, riding horses. Cool! Sweet! Sounds chill.
How do you turn that into a job? How about being a horse riding teacher? Starting up your own school? Who knows? Perhaps a business subject then. A Levels can lead you into all sorts of things. Want to travel the world? Pick a language to learn. Most people don't know what they want to do, just remember that you don't have to pick one job and stick to it. My mum went from Dentist to Stage Hand to PA for the Royal Family at one point. Now she's a chef. She actually failed her cooking A Level but she's a damn good chef and makes cakes for fancy functions.
My point is, you have no idea where your life is going to end up. If you still have no idea what subjects to take, just pick the one's you're good at. Can't really go wrong there!
Let me know how this goes :D