1: Write, write, write!
I think to improve your writing you really need to write something in English more or less every day. In my experience, it’s best not to be too hard on yourself, but just write a few lines about what you did or about your opinion of something. If you’re studying for an exam, you probably need to do more writing, perhaps writing the answer to an exam question at least twice a week.
2: Get your writing checked.
If you attend an English class, ask your teacher who will probably be happy to help.
3: Learn the correct version.
When you find a mistake in your writing, it’s a good idea to learn the correct version by heart. Otherwise it’s easy to keep making the same mistakes (I know!).
4: Keep adding phrases to your vocabulary.
You also need to keep reading and studying vocabulary to gradually make your language more sophisticated. I strongly recommend studying collocations as much as possible rather than single words when you’re learning vocabulary.
5: Take care when you’re writing.
Finally, I often see writing that could be improved by taking care about very basic things. Easy ways to make your writing better include making sure that you’ve used capital letters and full stops where you need them and checking your spelling. Also, it’s not a good idea to use abbreviations like ‘u’ instead of ‘you’ (except possibly in very, very informal text messages if you are a teenager).