My 10 top tips for A-Levels (from someone who has just finished them)...
1. Don't feel pushed in taking a subject you don't want to do, even if it might be useful or respected.
2. If you hate a subject after the first week, change it while you can!! It will only get worse, and probably an awful lot harder!
3. Make use of your free periods.
4. Do extra reading around your subjects.
5. Take general studies over extended project/ critical thinking, more universities (even some Russel Group ones) accept it than you'd think for most courses (obviously not medicine etc.), when many few take the other extras into account, even though they may be useful in securing your offer.
6. Ask a teacher straight away if you don't understand something.
7. Don't think A-Levels will require less work than GCSEs just because you are doing fewer subjects.
8. Work with your friends.
9. Don't be pushed to go to university even though you're doing A-Levels and everybody else is.
10. Most importantly, enjoy studying what you love!!
Sorry for rambling, just do what YOU think is best.... Good Luck