People get to the end of their degrees and have no idea what to do still. It's so common to not know what you want to do during school and college. Most people will end up in jobs they never really heard about - it's only when it's something so easy to see like medicine or law, do people really seem to be certain on things. Plus, having a complete plan at this age isn't that worthwhile, because you'll probably find you suddenly don't like a subject, or your feelings change. I was an absolute biology nerd in school, thought I had it all mapped out, until I got to A-level and realised I really disliked it as an academic subject.
Go with what you enjoy, so obviously do as well as you can with all of your GCSEs. Do pick to your strengths though - and if you like maths and science then don't be afraid to pick them at A-level. There's a whole host of careers in science, and lots of cool stuff to be involved in, even if you're not sure what you'd like to do in the industries. If your school or college has any links with outreach programmes from university science departments then they are really cool to see what it's like studying it further.
When picking your options, if you like lots of things, i'd start by ruling out all the things you definitely don't want to do. After that, you can look at the combinations and see what sort of paths it might take you.