Coleridge's The Rime of the Ancient Mariner is one of the most widely anthologized pieces of Romantic poetry for good reason; the archetypes, the symbols, the role of the narrator, the quirky protagonist - this poem has it all. There's a good study guide located at http://www.dundee.ac.uk/english/mariner.htm, including a concordance. A concordance is an index of all the principal words in a work showing location in the text. A concordance is useful if you're looking for patterns of words or recurring terms so that you can identify themes or symbols, among other things. This may be valuable to you if you are considering choosing Coleridge for the Unit Two Essay.
The University of Virginia maintains The Samuel Taylor Coleridge Archive, which contains, among other things, a critical essay on Coleridge and a dictionary that helps readers convert Coleridge's language to the present day (yay! some of that stuff is a bit tricky!). While the critical essay isn't new, it is potentially the only secondary source that I've encountered on the open access Internet qualified to appear in your paper 
Usually you'll rely on articles indexed within the Library's online databases 