Hey !
Firstly as you are in 12th, you need to complete your bachelor's degree in related field like fashion design or fashion merchandising, leather design, accessory design etc  and talking about the basic eligibility, a minimum of 50 percentage is required from a recognized educational board (in 10+2).  And obviously you must have a desire to learn things and should gain skills like pattern cutting and good sense for colour and shape. You will also need to spot trends. You must be artistic, creative and must work well in team. You can take up Bsc fashion designing. UG diploma fashion designing is also a Good choice. Do check the below link for more info.
https://design.careers360.com/articles/know-all-about-fashion-design
Hope this helps
All the best !

Hi satty

So if you want to pursue fashion designing then I will like to suggest start preparing a profile for yourself.

Various institutions that offer fashion designing as a course have entrance exams to take in students.

Nift is the best institute for this purpose. Pearl academy is another good institute. 

Since you are currently in class 12th focus on your studies and start preparing for the entrance exams you will HV to face after 12th side by side. You can refer to the link for better information:

https://www.shiksha.com/design/fashion-design/articles/prep-tips-for-nift-entrance-exam-2016-blogId-11660