The first episode that aired was The Man Trap, aka The One With The Salt Vampire. It was actually the sixth episode filmed, but NBC decided to air it first because they thought it was the most "exciting." Plus is featured a cool monster.
Most critics of the time weren't impressed. Variety said the show "Won't work," and that it was "An incredible and dreary mess of confusion and complexities." Which just goes to prove that critics have never known what the hell they're talking about.
The first episode came in at #1 in its time slot, probably because it was up against reruns of The Tammy Grimes Show on ABC and My Three Sons on CBS (believe it or not, there were only three networks in those days). Unfortunately in its second week, the competition aired brand new episodes, and Star Trek fell to second place, and two weeks later it was ranked 51st in the ratings. Which just goes to prove that the ratings system is flawed and meaningless, and doesn't judge the true popularity of a program.
I didn't really watch the show when it was first broadcast. Like most families, we only had one TV set back then, which my Dad controlled with an iron fist. He thought Star Trek was "stupid," so the instant it came on, he changed the channel to something else. I have some vague, foggy memories of seeing it a couple of times though (probably at someone else's house). I remember seeing a couple guys— one with pointed ears— walk into a room and disappear. I was way too young to understand what I was watching though.
When I was in junior high my local UHF station started running the show in syndication, and that's when I really discovered it. By then I was old enough to understand and appreciate the series, and it quickly became a huge part of my life. I've seen every episode more times than I can count, and bought enough tie-in merchandise to fill a modest-sized home (as one look inside my modest-sized home can attest).
I've written numerous posts about how Star Trek and its characters affected my life, so I won't go over that well-worn ground again. But I will wish Star Trek a Happy 50th Anniversary. Here's to fifty more!