serial=0,9600n8

Add the following kernel boot parameters in your bootloader

console=ttyS0,9600

Add the following line in /etc/inittab

S0:2345:respawn:/sbin/getty -L ttyS0 9600 vt100

And if you want to be able to log on directly as root in the console, please think to add ttyS0 in /etc/securetty