These links will direct you to some of the best open-access online guides for college-level expository writing:
University of Chicago, Writing in College: A Short Guide to College Writing, for general advice
Harvard College Writing Program, Developing a Thesis, to clarify your main argument
Trinity College Library, CiteSource: How to cite sources
Diana Hacker and Barbara Fister, Research and Documentation Online, including sample research papers that illustrate key concepts above (including how-to cite sources)
Grammar Girl, Quick and Dirty Tips for Better Writing, with a search box to distinguish between commonly misspelled words, etc.
Amherst College, The Writing Center’s Recommended Online Resources, for links to other sources