HI,

YOU HAVE TO contact with school authority first and tell them the reason of lost of original marksheet, if you have xerox copy of marksheet take those documents with you along with your transfer certificate , original marksheet, you need to apply for the duplicate marksheet, if applying for duplicate certificate can not done from school you have to directly contact with the education board from which you have completed  the 12th exam. also check the official webiste if any application form available for applying duplicate mark sheet.