There is two parts to getting a serial console set up under Linux. First, you must tell Linux to redirect its console output to the serial port. Second, you must set up mgetty to start a login process on the serial port once the kernel has completed booting.
Some distributions use mingetty for the video console, but mingetty has no serial port support.
You will instead want to use mgetty. Check your motherboard documentation for more information about this. Change this for the name of your kernel. You can use tty0 to get normal behavior, ttyx to put the console on another virtual console, or ttySx to put the console on a serial port. This is mostly used for passing options to the serial port.