If you don't know, go to uni. Make some connections. I didn't go, and I'm still OK with that decision, but sometimes I wish I'd gone just to meet more people. All the startups I've been involved with have started among a group of people who met at uni – and generally they're nothing to do with whatever they were studying. It's taken me until right on the cusp of 30 to realise just how important that phrase "it's not what you know, it's who you know" is.
My other piece of advice is to just say yes to more stuff. You won't know what you want to do until you try more stuff. For instance, I've been offered a couple of fun pieces of work off the back of doing a favour for my Dad a couple of weeks ago filming his charity cycle. Filming's not what I do, but I did fancy playing with a drone for a week. Now I'll be doing that for a couple more adventures by the looks of it. Point is it was a complete accident, so give yourself some more opportunities for 'happy little accidents' to happen.