The following organizations have a wealth of resources to introduce you to the nonprofit sector and find organizations to intern with.
Volunteer Fairfax is a long-time partner with George Mason University. Their aim is to connect local organizations with volunteer needs to individuals in the community (including Mason students) with an interest in serving others.
Connect Northern Virginia is an easy-to-use and customizable website for the Northern Virginia community. Launched in September 2013, it offers public and private sector information about a wide range of community issues and resources. It includes information and data connecting the Counties of Arlington, Fairfax, Loudoun, Prince William with the Cities of Alexandria, Fairfax, Falls Church, Manassas and Manassas Park and with the Towns of Dumfries, Herndon, Leesburg, Purcellville and Vienna.
Mason's University Career Services has an excellent collection of resources for students interested in exploring the Nonprofit Sector:
Charlotte Strauss (firstname.lastname@example.org) is the Industry Advisor for the Nonprofit Sector. Please utilize her expertise and advise.
Idealist.org is a nonprofit organization that helps you find volunteer, internship and paid positions in the nonprofit and government sectors.