hello,
you can get your registration number from your admit card. Look for the hard copy you took to the centre of examination. Else you have saved it as a soft copy you can get it from there. If not check the site where you have downloaded your admit card and get it from there. When you have applied there also you may get the number.

Take look and search for it. You will get it any way. Keep your important documents safely now on. else it will create  problem for you.

Hope you find it!