You cannot do 12th after just passing 8th class.first complete 10th class get your certificates then apply for 12th class in open school like NIOS

for taking admission in open school you need following documents.

• 8th-grade marksheet or a self-certificate
• age proof you can use Aadhaar Card or Birth Certificate
• address proof, and
• photographs.

Hope you understand!!

All the best..