This isn't necessarily about my first introduction, but its what drew me in:
I had seen the trilogy at some point in my youth. I liked the movies, but there were other movies I liked just as well.
When The Young Indiana Jones Chronicles premiered, I was interested enough to watch it and was completely hooked. The exotic locations, the action (at least in the second half of the premier), and the numerous elements of actual history made me an instant fan. I started looking up stuff on Lawrence of Arabia and Pancho Villa, and almost expected to see "Jones, Indiana" listed in the encyclopedia.
Anyway, after that I bought the videos, bought the books, a friend gave me the Marvel comics, and I've been a fan ever since.