NIFT

You will have to pursue a course from NIFT. Below are the details:
http://www.nift.ac.in/mumbai/ce/visual-merchandising.html

Yes, you can appear for NIFT entrance exam after completing your 12th.

Yes, to get into NIFT, you will have to appear for entrance exam.

Hi
B.design eligibility is 10 +2 from any stream but for B.F.tech you need PCM. so download the prospectus from their website. go through all the details carefully and apply.

Dear Saloni,
NIFT is one of the best college for fashion designing in India, No of famous designer's studies there it offers very good exposer and good skills enhancement support to students.
All the very best

If you are thinking of doing any diploma courses in nift then it's not much of a worth as you already have done your diploma. however if you are interested in lifestyle and accessories design/textile etc then you might give it a try. if you wanna do a graduation or post graduation then that is brilliant. and if you did Your diploma for three years you can also directly join from 2nd year onwards by lateral entry.