Grammar is important, I admit, but grammar through context will help a lot.

Make a good dictionary your companion. Always look up the words you know and the words you don’t.

While looking up a word, it is not only the meaning that matters; you will have to get to know: the meaning, the part of speach, usage notes (if any), synonyms, antonyms and most important of all is an example explaining the word.

It’s hard work, I admit, but you seem a serious scholar and want to get along with your studies. It’s worth, isn’t it?

Go to your school librarian and tell her that you would like to prepare for English and ask for a suggested reading list. She will know exactly which books will prepare you for the curriculum at your specific school.

Good luck