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
Harvard College Writing Program, The Harvard Guide to Using Sources, with rich examples in sections on Integrating Sources, Avoiding Plagiarism, and Citing Sources
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