I think you can start learning the very basic rules of writing and grammer, it will help you a lot to write correctly at first. the 2nd step will be leaning vocabulary that you need, the 3rd step, you have to read more and more Arabic texts. by reading texts, your writing skill will be improved so much.
A good exercise is to write some paragraph in the Notebook in italki, and I can help to correct them for you.
You are most welcome Yahia :)