If you're unsure about which adapter to purchase, the Apple Store can help you find the correct one. If you have a newer Mac, it will be a Thunderbolt (mini-display port) to VGA adapter. The official Apple adapter is $29.95, but you can also buy generic ones. A good place to buy a generic third-party adapter is monoprice.com ($14).
If you have an older Mac (one that is several years old), then it could be one of about seven adapters. You could bring your Mac by the Media Services office (198 building, room B27), and we can tell you which one you need. We also have this handy chart.
Again, you can borrow an adapter from the Law Library Ref Desk, 4th Floor. They have all flavors of Mac adapters, so we recommend you bring your Mac with you to see which one fits.