Try to read more and more English to improve your speaking and grammar skills. Read newspapers and also take help of some books and novels. But if you are preparing for some exam, then this is a lengthy task, so for exams take help of grammar books to clear the concepts like Change of Voice, Direct-Indirect Speech, Affirmative-Interrogative, sentences, figures of speech, etc. You can take help of Navneet Digest of your year, you can also refer to a boook called as English and Grammar by Wren and Martin.

Best wishes. Thank you.