Pololu makes a very versatile adapter for RS232, that provides hardware handshaking signals and generates the proper RS232 voltages (-12 and 12 V) from either 3.3 V or 5 V power: Pololu 23201a Serial Adapter Partial Kit
For interfacing with genuine RS232 devices, you really need to know what the device expects. In addition to the basic baud rate, handshaking can be in software (with start and stop transmission characters) or hardware -- involving any of several different control lines (RTS, DCD, DTR, etc.). There can be different numbers of bits per character (7 or 8 are common) plus possibly a parity bit, which can be even, odd or none. Finally, there can be 1 or 2 stop bits.