Hi there!
If you take a look through the links in
this thread, you'll find a whole bunch of responses to questions like these under the Context heading.
It might help to think of things in terms of the different
styles and
forms, since the styles (persuasive, imaginative, expository) describe the way you write, but the forms (essays, speeches, letters, editorials, feature articles, short stories, etc.) are what give you the structure you need.
So there's no such thing as an 'expository structure' since you might be writing an expository essay, or an expository magazine feature piece, and they'd both look quite different.
If you're still lost, let us know what style you're thinking of choosing and/or what form you'd be most comfortable with and it'll be easier to help you out