Are you replacing exisiting switches and lights or adding new ones? The short answer is, you use special switches called 3-way switches to control a light from two locations. The switches have no up side, so if the light is on when one is down the other is up and if you flip the one that is down up the light would go OFF and vice versa.
I can tell you how to actually wire this circuit, but I'll need to know where you will be bringing the power through, the light, a switch, etc. Also, is the light between the switches or not?