One of the first questions: Do you want to keep your options open?
Students often believe that by taking a mixture of arts and science subjects at Higher Level they can keep their options. While this may work for arts degrees, students will be less competitive (specifically see UCAS.com grade requirements for each subject at target) for broad-based science courses.
Second question: Are you inclined towards the sciences or maths? If you want to student a science subject or math you are should be advised to take at least two, and ideally three of Biology, Chemistry, Physics and Mathematics at Higher Level. This would take the shape of:
Some of the Higher Level subject combinations are more natural, such as Physics and Mathematics for a Physical science, and Biology and Chemistry for Medicine.
If you think you would like to study an arts or social sciences course at university but you are not sure which one, then Language A Literature, History, a Language B option, Classical Languages and Mathematics are good subjects to study at Higher Level: choosing one or more of these will provide a good foundation for your subject combination.
Other good Higher Level choices to combine with these good foundation subjects include: an additional Language B option, an additional Classical language, Economics, Further Mathematics, Geography, Philosophy, and sciences (Biology, Chemistry, or Physics). These subjects could be taken either in their normal subject groups or as a group 6 option.