I'd seen "The Motion Picture", "Wrath of Khan" and "Search for Spock" on TV here in the UK through the 1980s as a 6-7 year old, but they were little more than Bank Holiday viewing, instantly forgotten and probably not really understood in the first place.
Then, when I was 9 (1989) my Dad rented the TNG episode "Encounter At Farpoint" video from our local rental store and I loved it. However, the series was not due to begin on TV until September 1990. When it finally did, I loved it even more!
So "Encounter At Farpoint" was my first TV episode, and I suspect "The Motion Picture" was the first movie.