With that in mind, probably best to start by writing down your favourite subjects (if any). Make sure you take them because you may find after two years of studying for A-levels that you CBA to do the finance degree you planned on doing and so then all your A-levels that you took with the intention of doing finance are a bit useless if you've now decided you want to go back to law. Basically, don't pick your a Levles to fit a career - you've got to pick the ones you can tolerate and get relatively good grades in.
After you've written down your top four then you can think about the future. For example, if you wanted to do medicine/dentistry, you'd need at least two - if not more - science subjects. If you wanted to do economics, you may also think about maths because most uni's require maths as an A -level in order to do an economics degree.
Finally, consider that Uni may not be the best path. Degree apprenticeship? Straight into the world of work?
So I really, really advise you to do the same - you may have dentistry in mind, but if you don't actually enjoy doing maths and sciences, you certainly won't get the grades for them if you dread studying for them!