My best friend in 4th grade, Jim Caylor, lived on the other side of the neighborhood, where the cable company started laying cable, so he was the first one I knew to get cable. This was 1976.
We'd go to his house after school and we were switching around the cable channels, and on one of them we found Star Trek. He knew it, and his dad was a fan so we watched it. The episode was Tomorrow is Yesterday with the time travel element, I had no idea what this show really was.
After a while we started watching them every day, and discovered that the local Denver station played Star Trek right after the showing on cable.
By the time TMP came on the scene, I had seen most of the episodes, but I was hooked from that very first episode.
I've seen the first run on every series, although I missed a few here and there, and have seen every movie in the theater, most of them on opening night, and all of them except TMP, on opening weekend.