It depends on which exam you are taking, Academic or General. Both exams have the common writing task of a discursive essay on a topic of interest . This should be at least 250 words long with four or five paragraphs including a clear introduction, a paragraph stating each argument (along with relevant examples as evidence) and then a clear conclusion. The structure of the essay should be clearly laid out using signals such as "However", "Firstly", etc.
For advice on the other part of the writing exam, I need to know which type of IELTS test you will be taking.