Hello ananthakrishnan! It's good that you are thinking to do fashion designing after 12.
Here answering your question-To do fashion design course after 12th you have 3 options in hand with you.They are-
1.Just after your class 12 you can pursue your bachelor's degree in fashion designing and then further work in that field and make your career.
Degree course is generally of 4 year duration .
2.or else you can either pursue diploma in fashion designing which is generally of 2 years but it may differ from college to college.
3.The 3rd option which you have is you can do a certification in fashion designing which is of 1 year.
The admission process and fees differ from college to college.
Hope this helps you.