Music scores in the library can be found in OneSearch, just like books, ebooks, and other materials. The advanced search has a filter for "Material Type" where you can choose to limit your results to just scores.
Finding a particular piece can be tricky, especially when the title is something generic like "Sonata in C" or "Symphony in G minor." Different editions may have different versions of the title, or may be titled in languages other than English. Try different combinations of keywords, including the composer's name, the publisher if that's important, opus numbers or catalog numbers (like BWV or K. numbers), or names of instruments.
For many composers, the library has editions collecting their works in a set. If you don't find an individual score for a piece, you might find it as part of the collected works by that composer. You can search for collected works using the advanced search in OneSearch--search for the composer's name in any field, and "M3" in the call number. There are also reference works that can help you find the contents of collected works:
You can browse scores on the shelf or by using OneSearch's browse by catalog number feature. Scores, like books and other items in the library, are arranged according to the Library of Congress Classification. Take a look at Subclass M - Music to see what call numbers correspond to the genre of music you wish to look at.
If the library doesn't have the score you need, you can try other libraries just as you would for books. First try searching CSU+, and place a request there if you find it. If you don't find it in CSU+, try WorldCat.org. If you find something there, you can request it through Interlibrary Loan.