When you're researching music, a good place to start is your class LibGuide, or the Music Collection LibGuide. You'll find listings of the most useful databases, and you'll be taken through the library so that you get all the access the library provides. Another good first step is to make sure you're signed in to the library page.
Oxford Music Online is the online gateway for access to the Oxford Dictionary of Music, Grove Music Online and the Oxford Companion to Music.