A discursive essay, like most essays, should begin with an introduction and end with a conclusion. You can either talk about a topic neutrally, showing both the pros and cons, or you can argue for or against. If your essay is of the latter type, a basic structure would be to start with an introduction, then a paragraph for each of your points.
Imaginative or creative writing absorbs readers in an entertaining way. To succeed with this kind of writing you will need to write in a way that is individual, original and compelling to read.