Hi aspirant

If possible then change your subject.Take something else.The easiest are hindi and sanskrit.If you still want to study CS as an extra subject then don't leave any classes.Attend all classes in your school.Try to learn c programming .Wherever you find any difficulty ask from your teacher that you are unable to understand a particular topic.

See the most important thing is how attentively listen to your teacher.Most of the students face the same problem but once they try to understand it becomes quiet easy.So try to learn basics at first like syntax of C,why using semicolon,using #include statement.Try to write basic program of addition ,subtraction,mod etc.

You can refer book by Sumita Arora for class 12.

Hope this was helpful