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.
Writing is one of the five language skills, which are:
When writing, it is essential to understand the basic system of a language. In English this includes knowlege of grammar, punctuation and sentence structure. Vocabulary is also necessary, as is correct spelling and formatting.
When writing it is also very important to know the difference between cohesive and coherent. A text is cohesive if its elements are linked together. A text is coherent if it makes sense. It should be clear that these are not the same thing. It should be clear that these are not the same thing. That is, a text may be cohesive (i.e. linked together), but incoherent (i.e. linked together), but incoherent (i.e. meaningless). Here is one such (invented) text:
Maria is my sister. My mother usually goes shopping. When I am tired, I usually take a bath and rest for a while. Well, that's all.
Each sentence is notionally linked to the one that precedes it, using both lexical and grammatical means, but the text is ultimately senseless - to me anyway (and I wrote it!).
A writer may write for personal enjoyment or use, or for an audience of one person or more. The audience may be known (targeted) or unknown. Taking notes for study purposes is an example of writing for one's self. Blogging publicly is an example of writing for an unknown audience. A letter to a friend is an example of writing for a targeted audience. As with speaking, it is important to consider your audience when writing. There are many different styles of writing, from informal to formal.
Some useful and practical tools and resources have been collected so far. Just click on each of them to improve your writing.