Serial communications require a minimum of three signals to operate. More if you are going to use hardware flow control. If you assume you do need hardware flow control you can't go wrong.
Gnd, Tx and Rx. Get the pinouts of each end and determine if each end is DCE or DTE.
If the ends are different, you need a straight through cable. If both ends are the same (and it doesn't matter which both) you need a null cable. A little investigation of your connections will reveal which cable you need. Look specifically for the Tx and Rx pins.
There is an excellent description here: http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html#anchor242192