Start your research with an encyclopedia article from Oxford Music Online, MGG Online, or RILM Music Encyclopedia to get a general overview of your topic and start your list of potential sources from their bibliography. Also check out Taruskin's Oxford History of Western Music. He touches on a lot of topics and can help provide context.
Now that you have a solid background understanding of your topic, it's time to get more specific sources. Academic Search Complete is a general database to start with. Select "choose databases" at the top of any EBSCO Host database to see others. Education Full Text and ERIC are good ones for education while Music Index and RILM Abstracts of Music Literature are better for general music research.
Tip: When looking for a specific piece, check out composer and publisher websites. Many post partial or full recordings of their works. YouTube and Spotify are also great free resources.
Along with these resources, check composer and publisher websites. They may have study scores available for viewing.