Class 11

You must complete your 12th first.

Hello MA'am
CLAT Eligibility: Academic Qualification A candidate must have secured minimum marks required in 12th standard, as per the category to which s/he belongs: For Unreserved /OBC /Specially Able Persons (SAP) and other categories -A minimum of 45% marks required. For SC/ST Category -A minimum of 40% marks required.

B. Des. in Fashion Design is a 4-year undergraduate course, the minimum eligibility for which is a Higher Secondary Certificate (10+2) qualification with a minimum aggregate score of 50%. The program is spread over 8 semesters, with the final semester being based on the candidate's chosen stream of study.

Refer the link:
https://www.shiksha.com/exams/ntse/syllabus.

It totally depends on your interest. If you want to have a career in management then opt for BBA, otherwise you can go for BCA.

CAT or any other qualifying exams are totally based on quantitative aptitude, data interpretation, Logical reasoning, verbal. Maths is an inherent part of these exams. It is not about is it advisable for somebody like you mentioned to go for CAT or not. Rather it is about whether you 'want' to get into Iims or not because CAT can get you into IIM.

The CPL training eligibility requires you to have maths as a subject for admission. Therefore, it would be a good idea to pursue maths.

Many options are available for you. Since you are in class 8th right now, you should introspect and check what are your interests. Options available to you:-
If you are an ICSE Student then you will have to choose a stream in 9th Class. If you want to know more about ICSE then kindly let me know in the comments. Assuming that you are a CBSE student, you need to choose stream in XI Class:-
Medical
Commerce
Non-Medical
Arts/Humanities
Make a wise decision and remember CBSE has revived its Class X Boards from 2018 Onwards so you will be included in that.