My Avalanche truck will not start. What could this be? on 2004 Chevrolet Avalanche 1500

The lights are coming but very dim. When I press the button to unlock or lock the truck, lights are fadingly dim and the horn sound sick in trying to sound off....when I turn the key in the ignition, its a little clicking noise...What could be the cause of this?

Asked by for the 2004 Chevrolet Avalanche 1500