The port C pins are digital I/Os (you don't have to do anything special to use them as such). It just so happens that you can also internally connect them to the mega8's ADC. For example, it's possible to use the ADC to read the analog voltage on pin PC0 while using PC0 as a digital output. The result probably wouldn't be very interesting or useful, but you could do it.
So in short, if you want to use the pins on port C as digital I/Os, do the same thing you'd do for the pins on port D or port B. If you want to use them as analog inputs, set them as digital inputs and configure the ADC module appropriately.