CBSE 12th

12th does not come under the purview of UGC. So I guess you are safe in that regard.

Hey Deepan
Application Procedure:
The registration for admission will be made via National Board for Higher Mathematics (NBHM) & Ph.D scholarships Screening Test. Crucial details will be publicized soon. Only the applicants who are best suited as per the criteria of eligibility should apply.

Use this link:
https://www.shiksha.com/college-predictor

Use this link:
https://www.shiksha.com/college-predictor

You can get the papers from the market and they are also available in the university library.

Admission to the second year of Diploma is not feasible.

Eligibility criteria for the post-Higher Secondary Certificate (HSC) Diploma course is an HSC pass certificate. You are eligible to seek admission in Diploma course from a Polytechnic. Legally it is permissible to do Diploma and degree simultaneously. You can not pursue two degrees simultaneously from same or different universities. You are advised to evaluate whether you can handle simultaneous studies load of two courses and challenges in case of conflicts in time table, examinations etc.

HI Manisha,
You don't need to appear for any entrance exam. Although, different colleges and universities conduct their own admission tests for B.Com.

CA CS CMA, BCA BBA BMS, B.Com (Hons).
All these are available for you. Choose according to your strength and interest areas.