(via genmon)

• The first words sent by tachygraphe (optical telegraph, or Chappe telegraph), by Claude Chappe, 1791: “If you succeed, you will bask in glory.”
• The first words sent by electrical telegraph, by Samuel Morse, 1844: “What hath God wrought!” – however, this was the first telegraph using the repeater system. The actual first message, on the demonstration system, was sent with no repeaters, for just 2 miles, and was received by Samuel Morse but was sent by Alfred Vail in 1838: “A patient waiter is no loser.”
• The first words spoken on the telephone, by Alexander Graham Bell, 1876: “Mr. Watson—Come here—I want to see you.”
• The first characters sent on ARPANET, the predecessor to the internet, by Charley Kline, 1969: “lo” for “login,” but it crashed.
• The first words sent by text message (SMS), by Neil Papworth, 1992: “merry Christmas”
• Here's a good one, from this BBC article on various first words. The first words spoken on YouTube, 2005: “Alright, so here we are in front of the elephants.”
• Related: fictional first words spoken on Mars, a list. The one I always remember is from Red Mars by Kim Stanley Robinson, the words of John Boone, 2020: “Well, here we are.”