I was introduced in 1977. There are two memories and one activity which could have been the first inkling.
First, I remember hearing Uncle Jeff describing D&D to my parents. He is who introduced me to hex and chit wargaming about a year earlier with Starship Troopers and Kingmaker, both by Avalon Hill. In the intervening time I had also found SPI games and purchase Star Force and World War 3.
Second, I remember seeing an ad in Starlog magazine. It was for a game store and also included Cosmic Encounter and Lou Zocchi’s Star Fleet Battle Manual.
Finally, the activity was being a member of the Golden Triangle Conflict Gamers, a wargame club in southeast Texas. I know D&D was played at the club, but the first memory I have of it was in early 1978 after I started playing. I joined into an expedition when one player left, taking his character.
As for what was first, I think it was my uncle’s talking about the game. I seem to remember this leading me to look for D&D in both those ads and at the local game store, Toys by Roy.
Between Christmas and New Years in 1977 I used Christmas money to buy the Holmes Bluue Book and a first edition Gamma World boxed set. There were no Holmes boxed sets left after the Christmas rush or dice so I bought the Gamma World set to obtain the latter.