you should read more.... the 1st time you may catch little
but 2nd time you should catch more....
.... after you read more and you feel catch more..... ( i hope you will not
feel sleepy )
then you can try to have some exercise ( test or old paper ),
then you will find some questions that you dont know the answers,
then you can tru to find the answers in you text book ,
if you still not understand, you can ask you classmates/teachers thanks
if you follow this method, i believe you should have at least Grade C in public exam thanks