Oxford Music Online includes the Grove Dictionary of Music and Musicians plus other trusted music reference works. It's a great place to start exploring musicians, composers, and genres, with entries written and reviewed by experts. Many of the entries include bibliographies that can lead you to even more great sources.
RILM is an excellent bibliographical tool for finding information related to journal articles, dissertations, conference proceedings, books, and more. If you find materials in RILM that you think would be useful for your project but RILM does not include the full-text, reach out to Kathy Essmiller, OSU- Stillwater's music librarian at kathy.essmiller@okstate.edu for help.
JSTOR is a trusted database for scholarly journal articles, books, and primary sources across many disciplines, including music. JSTOR may not have the most recent issues of journals, and it only includes a portion of the music-related literature and research that have been published. It is a good place to start your research, but it shouldn't be the only place you look. You can search by topic, author, or title. Many of the resources are available for download.
Music catalog covering all major genres and time periods from medieval to contemporary, from choral works to symphonies, operas, and the avant-garde.