Sometimes you know exactly what piece you are looking for. Suppose you want a score of Felix Mendelssohn's Concerto for Violin and Orchestra in E minor, Op. 64. In searching for pieces with generic titles, such as concerto, symphony, sonata, etc., it is helpful to make use of the opus number in the search. Go to the Libraries' home page at http://www.lib.ua.edu. Click on Libraries' Catalog under Resources on the home page; then click on Advanced Search. In the first box, type the composer's name, last name first: mendelssohn, felix; choose "as a phrase" and "Author" in the drop-down menus on the right-hand side. Notice that you do not need to use capitalization to search the catalog. In the second box type "64" and leave it as a keyword. Find the drop-down menu for "Type" below the search boxes and select "Music Score." When you click Search you will find that the library has several scores of this concerto, including a miniature score and a score of a piano reduction with a violin part.
Remember, it can be tricky to find the score you are looking for, so if you need help don't hesitate to ask for assistance!