you need to submit undertaking that you belong to following category and you will be able to avail that certificate before whatever the next date priority is set or at the time of admission.  after that you may go futher to complete your form compilation

the below link is for college prediction you may go with that and find best possible college that you may get based on you rank and category;

https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer

good luck

You can contact NTA on there official contact information provided below:-

https://www.nta.ac.in/ContactUs

They will definitely help you.

all the best