The first thing to consider is whether you will be installing a digital or analog dish. If you are using a digital one, steve is right, you must have a clear view of whatever part of the sky the provider dictates. Analog however is a little more forgiving and can "see" through obstructions like trees and such.
A digital dish is one of the small ones, about 18 inches in diameter.
An anaolog dish is much larger sometimes 12 feet in diameter