Hello

Don't worry . Firstly go and check you register e-mail they have send mail at the time of registration.  It contains your application number.  Check spam folder also.

Secondly you can check you messages inbox they also send message which contains your application number while you are registering.

Other wise go to that government website from which you have  register for exam. There go for forgot username or application number ,in that section they will ask you  question and then you can get your account.

Good Luck.