Well, that depends on where you live. Here in the greater Philadelphia region it is generally channel 3. If you want to find out what it is for your area try the TV Guide section in a newspaper or on your TV Guide channel.
CBS broadcasts on many different channels. It just depends on where you live and whether you have cable or satellite television. For me, CBS is on channel 7. You can find more information here: http://www.tvguide.com/