If you are using another computer as a terminal into your FreeBSD system, get a null modem cable to go between the two serial ports. If you are using an actual terminal, see its accompanying instructions.
Then, modify /etc/ttys (see ttys(5)), like above. For example, if you are hooking up a WYSE-50 terminal to the fifth serial port, use an entry like this:
ttyd4 "/usr/libexec/getty std.38400" wyse50 on secure
This example shows that the port on /dev/ttyd4 has a wyse50 terminal connected at 38400 bps with no parity (std.38400 from /etc/gettytab, see gettytab(5)) and root logins are allowed (secure).