Arduino Mega's auxiliary serial ports
You can indeed use a serial monitor on your PC on the Mega's first serial port, in fact you will have to if you are connecting to one on the PC via USB port.. And just as with the Uno, you can connect a TTL serial device (like RS-232 serial, but using logic levels of 0 and +5v instead of [-3v - -25v] and [+3v - +25v] (which most PCs implemented as -12v and +12v).

If you need to connect to a true RS-232 device, you'll need to convert the TTL logic levels to something within the RS-232 spec; -5v and +5v would probably be the easiest.