I'm keen in writing stories of my imagination. However, my Grammar is not up to the mark. Even I use very simple words and not complicated words like the ones who use in Novels. I read many great books like 'To Kill a Mocking Bird', 'Gone with the Wind', 'Kane and Abel', Atlas Shrugged, The Fountainhead and many more. I understand them very well, but while writing I make grammatical mistakes and I use simple English like we use in School. So please suggest me some tips on how to improve my English. My native language is not English, but that doesn't mean that I should not be proficient in English, isn't it? Thank you.
TL;DR - Read. Learn. Write. Finish what you write. Never stop repeating the cycle.
Reading and writing is a cultural conversation. People write books to say something. Other people read those books and a few of those write books in return.
As Rhyous has pointed out in their fine answer, you should read. You want to write books and stories and contribute to the cultural conversation? That's great! Read a variety of books in different genres, from different eras. Pay attention to what you've read, internalize it. This is exactly how native speakers learn the language.
You'll find that books build off of each other. So read books that you enjoy. Read books from genres you loathe. (Well, skim them at least.) When things confuse you, ask a friend, what the hell is going on in this book? The answer may lead you to another book.
You'll get, from all of this reading, a feel for tone, pacing, and story. You may even enjoy the reading! (It is important this not feel like a chore.)
All this reading will show you how words are put together, and probably help increase your vocabulary at the same time.
I'm stressing this section because you're not a native speaker, but it's applicable to all.
You do not need to be a grammarian to write. But you do need to learn to put a sentence together in a logical way. Most writers pick up grammar by osmosis, through reading.
Read through a good style book like The Elements of Style. I take that back, read that book. It's very short and is filled with (mostly) good examples of simple writing.
There are good books on writing out there. My favorite is Stephen King's On Writing, but there are many others. The best ones are short and to the point and fun to read. (If the writer of the book can't keep you turning pages, why would you take their advice?)
These books will teach you in a formal way what reading will show you: How to structure a story so the reader cares what happens next, how to make characters the reader would take a bullet for.
At some point, you'll want to write. You won't feel that you're ready, but you should write anyway. Accept that you will suck for a while. That's okay, all you have to do is write something better than the last thing you wrote.
Actually, you could start writing right now. Why? Because there's an adage: We all have thousands of bad pages inside us, or dozens of pad paintings, or terrible songs. The goal is to get them out of us by creating until our ability catches up with our taste.
Get feedback from people whose work you respect: Reading groups, editors, beta readers, and agents. ("Feedback" from friends and family is usually just friends and family discharging an obligation.) Revise.
Important: Finish what you start.
Believe in yourself. Keep writing, no matter what.
Read. Read some more. Then read. Then Read some more.
I have read over 200 books on my "Read" list on GoodReads and I haven't even added them all. In comparison, others have read a lot more.
Read in English of course. Just read two books a month, and in a year, you will be at 24 books and in 5 years you will have 120 books.
The more you fill your mind with higher quality grammar, the more your own grammar will improve.
Studying grammar will help, but not without the reading.
The only thing that will help more than reading is reading out loud.
Several people have said the basics here (read, read, read) but I would add that you want to target your reading at the writing you want to do, English is not a highly formalised language with a single grammar convention, or even spelling set. I'm a native English speaker but due to some,... interesting... let's be charitable, some interesting policy decisions by the government about the time I started school my reading and my written English were woefully behind that of my classmates for many many years though I excelled them in verbal communication. It wasn't until I was taught to read well and fluently that my Written English began to catch up. These days my written English is very good for a variety of uses but I find that when I write fiction my style is heavily influenced by the authors I am reading at the time, which means that when I've been reading a lot of Lovecraft or Tolkien I throw out a lot of first draft as bloody unreadable, so tailor what you read to the kind of writing you want to do so you get the nuances you want.
I'd also like to add that in my personal experience pen and paper beat the hell out a word processor for really getting a feel for how a piece of writing sits on the page and how it comes off the page to a reader. Always, always, always, but always get a living human being to do your spelling and grammar checks, machines can't cut it.
MS Word has spelling & grammar checker. It also compiles readability statistics & lets the user chose writing style, i.e. casual, standard, etc. The app isn't 100% reliable but it gives you a starting place.
A site you might find helpful is the Perdue Online Writing Lab [OWL] @ https://owl.english.purdue.edu/. Most of the information is grammar related.