It depends on what you like, what you're good at, and what you might want to do in the future. Generally, try for a variety of subjects to keep things open down the line (e.g. geo, latin, computer science - lots of different skills and routes they could take you; or history, computing and business) and consider what A Levels/ career/ degree you think you might be interested. If you're not sure, then have a little think about it.
You don't need to know for certain, but if you think you might like to do something like economics, then business studies and geography would be useful. If you'd like to go down the medicine route, then Latin and History would be surprisingly useful.
Overall though, prioritise what you enjoy or think you will. If you have your first few lessons and hate it, most schools will be willing to facilitate a change in decision for the first few weeks, so try not to stress too much. Good luck!
Edit: I took all bar business out of those, and am now taking Economics and Geography (plus Maths) at A Level, so feel free to ask about any specifics - I might be able to help ^^