I use a MAX3223 RS232 Line Driver/Receiver. This is a single voltage chip that produces the ±12V levels necessary for the RS232 standard. In other words, you won't need to provide any voltage levels other than Vcc. Use this chip to interface Tx and Rx on your µ-controller to the matching pins on a standard DB9 connector. The MAX3223 is a TI part and you can get the data sheet on their website (wwww.ti.com).