For Indian Foreign Service (IFS), you need to appear in Civil Services Examination that is conducted every year by UPSC.
As IFS is one of the top All India Services, you need to get good rank in the merit-list to secure your seat in IFS.
It doesn’t matter much what subjects you read in school (10+2); you have to look at the syllabus for Preliminary Examination that comprises of General Studies, Aptitude Test as well as Main Examination that has 4 papers of General Studies, Essay and optional subject.
However, NCERT books are the best resource to begin with as these help you in clearing basics. So, whatever you study build better understanding so that your concepts get clear.
Right now, don’t think too much about this examination and just keep gaining knowledge and broaden your awareness level so that by the time you become eligible, you have a solid base.