I am new to anything vintage computing and would like to build a simple project with my Macintosh SE. My Hello World goal here is to simply show a string of text received from the serial port on the Macintosh SEs display, and I'm not sure where to start. I gather that I will connect to it with a null-modem serial adapter, but how will I receive the data sent over the serial on the Macintosh? Is there a Terminal application in the OS or anything that can monitor the serial port? Any pointers appreciated. I'm looking for an explanation of what, other than connecting the cable/adapter, I need to do to use the serial port for communication and what is the simplest way to get my feet wet and just prove that a message can be received over serial.
The OS itself has no terminal application by default. There have been multiple terminal programs available in earlier times, though. The most prominent and easy to use might have been zTerm. Double-click with shift, choose port, set serial parameters from the popup-menus at the window bottom and you're good to go.
Search for zTerm in your favourite old Mac software collection, such as InfoMac or UMich.
Apart from the correct cable, switching the symmetric RS-422 Port to RS-423 mode by earthing one pin (can't remember which), there was much fuzz in earlier times about "hardware handshake cables" — which might be worth consideration when you want to seriously transfer data. The Macintosh lacks some pins to provide the necessary "inner" (RTS/CTS) and "outer" (DTR/DSR) handshaking to signal the peer to pause sending data, end the connection, etc.
One of the earliest Mac program to access the serial ports was MacTEP, a Basic program that ran using Microsoft Basic 1.x, to poke the correct baud rate settings into the SCC chip and then peek and poke at the SCC data register to input and output serial data.
Any programming language you can find that runs on a Mac SE (Consulair C or MacForth, et.al.) could probably do likewise.