I recommend first watching the feature film, starring Kurt Russell and James Spader, then SG-1, then the two SG-1 movies, then Atlantis, then Universe.

Elevator pitch: the ancient Egyptians were enslaved by a race of alien beings who ruled by posing as gods with their advanced technology. The pyramids were their landing pads. The Egyptians eventually revolted and the aliens fled in ships, leaving the Egyptians to bury the Stargate, which is an advanced portal that lets people cross instantly between worlds through a wormhole. In modern times, a dig discovers this Stargate and the American military, with the help of an archaeologist disgraced for his theories about ancient Egypt, figure out how to use it to travel to other worlds themselves.