All you need is a level converter circuit to change the 3.3V logic level up to the +/-12V RS-232 standard levels, and the serial port connector itself. Here is one description of how to do it with a Maxim MAX3232 chip: Adding a serial console to the Raspberry Pi
You can buy such a module prebuilt from Ebay for example:
http://www.ebay.com/itm/RS232-Serial-Po ... 0438518755
there are cheaper ones, but be sure to get a chip that will accept and provide 3.3V logic levels to the R-Pi, which does NOT have 5V tolerant I/O pins. I believe the cheaper MAX232 based modules use 5V logic level, instead of 3.3V.