If you want to get a transfer certificate after 12th then you need to visit your previous school and enquire that directly how to obtain a TC they will guide you better or you can contact the school administrative office by phone call  or by email.required documents can be the Aadhar card pan card fees clearance certificate and leaving certificate .may be required to write an application for formal application for address to the principal . the processing time varies from school to school so ensure that you apply to apply to the school very advance.then keep following up with this in school.