Students who are learning English often learn to speak first, and then they want to learn to write in English.
Yet, the question to settle is:
What is the best way of achieving this?
Well, assuming you can write at least basic sentences, there are 5 major points to take into consideration if you want to improve your written English.
The first point to remember is that grammar is more important when you are writing than when you are speaking.
Usually, when you write something, it is for a particular reason, for example to apply for a job or write a letter of complaint.
In these cases you are trying to create a positive impression.
Verbal language is changing more quickly than written language, so there are instances of "street language" where even native English speakers use incorrect grammar.
This isn't the case in written language, where the language is usually more formal and more structured.
So you really need to practice your grammar and do lots of exercises to improve your knowledge and use of grammar.
This can be done by taking a class, by hiring a tutor, or by self-study using the internet, books, podcasts, or CDs/DVDs. There are lots of options available.
Second, if you have a limited vocabulary, this will become more evident in written English because your words are concentrated together in one place.
So try to increase your vocabulary to improve your writing.
There are many ways to do that, but you may need to try a few techniques before you find one that suits you.
Third, you will not be able to improve your written English unless you have someone check your work.
Write as much as you can and ask someone to look at it. Whether it's a teacher, a co-worker or a friend, ask them to look at your work and show you any mistakes.
This is very important, since if you don't know you are making mistakes, you will not be able to correct them.
Fourth, there are different styles of English. Different registers (levels of formality) apply to different kinds of texts and this makes writing English difficult sometimes.
To learn to write in English you really need to become aware of these styles. There are books to help you if you do not have an English teacher.
The next point will also help with this.
Fifth, one of the best ways to learn to write in English is to read as much as possible, in as many styles as possible.
The more you read, the more different styles of writing and the more vocabulary you will be exposed to.
You will start to know intuitively what feels right and what doesn't seem right.
It's not really something you can achieve in immediately, but with hard, efficient work and gradual improvement you should definitely get there.