How do I shut off the gas line that comes into my house at the gas meter?

There is usually a gas valve located before the meter at the gas riser pipe that comes out of the ground or if propand you turn the valve off at the tank. When all else fails call the utility company.

A lot of gas shutoffs have an ear, so it can be locked off.

Here is a schematic example of one without:

http://www.png.ca/images/turnoff.jpg