CA

Course wise difference is already known to all.
Basically, a chartered accountant is someone who has expertise in Taxation, accounts and auditing. On the other hand, a company secretary is one having expertise in all the laws, rules and regulations.
Numbers/taxes or law/company affairs
Both have different work fields and opportunities. A time was there when CA was more remunerative then CS. But the change in time and induction of new laws have widened the scope of CS too and therefore more or less they both stand at equal levels in case of employment.
However in practice, we need to remember that anybody and everybody can approach a chartered accountant for basic accounting or Taxation work but same is not the case with company secretary.
CA is for the masses.
CS is for the classes.

You can only begin preparing for CA, once you begin with your 11th grade (I am assuming you will opt for commerce stream).
You need to focus on your 10th standard subjects as of now as there is hardly anything you can prepare in 10th standard that is going to help you in your CA exams (except maybe a little bit compound interest, simple interest).
While you are in your 11th and 12th grade, you need to just put in some extra effort in certain subjects that will further help you sail through CA common proficiency test (CPT). This is considering the fact that there is a change in examination pattern from objective to subjective.
In 11th grade, just keep your basics clear about subjects like accounts, statistics, mathematics and economics.
In 12th grade, you are required to put more emphasis on certain subjects namely accounts, economics (Micro+Macro) and mathematics (calculus). If you do have time to spare, you can go through the basics of Indian Contract Act. Lastly, do not forget about class 12th exams and devote time accordingly.
After you are done with class 12th exams, you have 3 months before your CPT exam (give or take a week). You just need to plan a routine for those 3 months and you will get through.