The writing section has two different parts with different requirements. Part one asks you to summarize a graph, chart or diagram. The easiest way to approach this task is to try and imagine you are explaining it to your parents or grandparents. They have found something in the newspaper and want you to tell them what is important about it.
You should begin with an introduction: “This chart/graph/diagram shows…” and then a paraphrase of the text in the question. Try to avoid copying words from the exam sheet because they will not count toward your total of 150 words. After your one-sentence introduction, you need to choose three pieces of information to describe: something changed, something stayed the same, something is very high or low, etc. Just find three interesting features of the graph and describe what you see. It is very important that you do not give your opinion in part one. The question asks for a summary, not an interpretation. You do not need a conclusion in part one; when you are finished describing your three main points you can stop. Make sure you write more than 150 words (remember that words copied from the question page are not counted as your words), less than this will result in a severe penalty so write about 170 words to be safe.
Part two of the writing test is the one that is more familiar. You will be presented with an opinion and asked to agree or disagree or you will be asked to discuss two opinions and give your own point of view. Make sure you use paragraphs, have an introduction and a conclusion, include all of the information required in the question (for example, if you are asked to discuss two opinions and you only discuss one then you failed to complete the task) and clearly state your opinion in the conclusion: “That is why I feel…”, “I think that _______ is the best way to…” Again, you must remember to write enough words, less than 250 words will be penalized so write a 10-20 extra just to be safe.
In general, you should be confident, creative and expressive with your writing. IELTS does not want you to be afraid of making grammar and vocabulary errors, they want you to show them what you can do. You will get a higher mark for attempting a complex grammar structure but making a mistake than you will for using basic grammar structures without errors. Of course, you will get the highest marks for using a range of grammar structures correctly, but you will still be rewarded for the attempt.