Need help with Illustrator creating curves and filling below.

[benjamin.morgan]

http://www.freecsstemplates.org/preview/unilluminated/ I like how their header thing curves. I want to create a header in illustrator with one curve. I can make the line but I am not able to fill the color in below. Maybe a blue in the top with a silver color below the curve.

 

[artimes]

Have you tried this tutorial - http://robcubbon.com/designing-waves-and-wavey-lines-in-illustrator/