Hi

You have to carry the same ST Certificate that confirms your caste category at the time of admission through JOSAA.

Firstly, while filling the form of JEE Mains you must have uploaded the scanned copy of your certificate that is issued by the government.  also, while registering for JEE Advanced, you must have been asked to upload your category certificate. The format is already given for SC, ST and OBC castes in their official website. Fill that and upload.

Best of Luck.

Hope this helps.

Cheers!