1) Accountant is a broad term and their are many different areas that you can practice in e.g audit, tax
Some work within companies and work internally while others are part of external firms and go in tocompanies to review their accounts for them. Again you'd be able to find better definitions on google.
2) Pay depends if your a trainee in a good city firm you can expect to start off on about £23,000 which will increase as you complete professional exams. However if you are in a smaller firm the starting trainee salary will be less. A newly qualified would probably earn something over £30,000 with higher wage as experience increases.
3) My sister is a trainee accountant and she did Maths, Geography and Business Studies. I think Maths would be very beneficial however I don't think it entirely matters much about the other subjects you take. Just take things you enjoy unless you plan to go to a top uni where 'academic' subjects are preferred.
Hope this helps!