we were not taught stereoismoerism in class 11 so should I learn them before starting class 12 organic for class 12 boards

If you are preparing for class 12th board examination then you don't need to learn stereoisomerism before learning class 12th organic chemistry. Structural Isomerism is sufficient for understanding class 12th Organic Chemistry. But If you are preparing for competitive exams then you need to go through whole 11th class organic chemistry syllabus.