For filling the form u do not need marksheet .But yes ,if ur shortlisted (based on JEE MAIN RANK/MARK) u will have to produce ur mark sheet before the SSB .If u do not have it u can produce a bonifide certificate and attestated copy of 12 marksheet.U have to specify the reason for not having the marksheet.If u have given it in ur college /job ,u have to get the bonafide from them.If lost contact ur school and get it from them.If lost u have to register a complain in the police station and have the letter ,it will help.