You need a master's degree from an ALA accredited school for some library jobs, but not all of them. You generally need a bachelor's degree to get into a library master's degree program, but it does not matter what subject you majored in in college. My library school had people who studied everything from accounting to ancient Greek.

The American Library Association has some more information about library education and library careers. You may also want to browse LISjobs.com to get an idea of what kinds of jobs are out there and what they require.