English grammar is a vast and profound topic in itself. But what i conclude from your question is that you are confused where to start from or you are confused to divide it into the categories the basics and topics to be studied after the basic.
So ,here are few topics.
1.sentence (you should have proper knowledge of sentence,its parts, division,framing and types).
2. Clauses(to advance and enhance your sentences that means to make complex and compound sentences)
3. Verb forms (its easy because you have to learn it ,thats it)
3. The most important part of the English grammar is tenses. ( There’s no need to describe why! )
4. Parts of speech ( noun,pronoun, adjective,adverb, conjunction, preposition, articles) No,thats not important to read them very deeply,but the basics about all those parts of speech, and how to transform a word feom one part of speech to another
5. Subject verb agreement. ( These are almost 15 rules,very easy to learn. The reason is not every verb goes with every subject)
6. Modals (apart from helping verbs,these words are used in the sentences)
Now comes the second part, a bit advanced than the basics:
9.. Active passive
10.. Reported speech