I suggest you to read as much as possible (look for essays on internet and concise articles) and then write some practices at home. Before writing anything brainstorm some ideas, it will help you to create a structured text.

You need sources of information to get ideas in order to write a text, that's why to read constantly is very important. But don't go crazy! choose a topic, read something about it and then write your ideas about that specific topic. If you don't know how to say something google it, it will also help to increase you vocab!

Good luck!