Structure is important for any written assignment. Without a clear structure your ideas and work are likely to be jumbled and unclear, so even if they are good, you may not get the credit you deserve.
The first step is to spend time familiarising yourself with the brief you have been given and any other information your course provides about organisation and structure. Some written assignments, like research reports have fairly rigid expectations about the structure you should use; others will be more flexible.
However, all academic writing will have an introduction, a body and a conclusion, for example:
Always check your assignment brief for guidance around specific requirements. The Study Guides, provide more general guidance about different types of writing assignments.
