You dont qualify to answer GATE after 12 std. GATE exam is written by aspirants to get admission into M. Tech or jobs into PSUs. After 12 std to get into engineering i.e B. Tech/ B. E you need to answer JEE MAIN and ADVANCED to get into IITs, NITs.
Or you can answer any state common entrance test. You can also take admission into private university by answering their own exam.
After completing B.Tech you can answer GATE