IMO the best way to do things (how I did them) is to think roughly what subjects you enjoy doing. All the experts agree that it is dumb to do qualifications (in your case A-levels) that you will find hard or boring. Do the ones you enjoyed at GCSE and/or got good grades in. If those happen to be P.E, R.E or something like that - try again.

However, think ahead. If you want to go into a science at university you need an A-level in it and if you want to do something in the business sectors any business related A-level will be invaluable. I speak through personal experience as a 1st year student.
It is best to decide what you want to do (roughly) sooner rather than later as it means you can plan ahead and get to do what you want to do for a good salary sooner. Not essential at all of course.

In my case I was interested in becomming a Chartered Accountant. I decided to take A-level Accountancy, Law (because I am interested in anything legal) and economics (because I like stuff to do with the economy such as interest rates etc). After I got into my A-levels I had so decided on what I was doing that I didn't apply for any other subjects other than Accountancy and Finance in my uni applications.
I'm now deep into my first semester at uni. It's hard work, but great fun and university is just so cool.

You don't have to go to uni, but it is becoming more and more difficult in the Uk to get a well paid job without one and I don't bet how hard its going to be 20 years from now (when you and I will NOT want to go back to school)!