The first time I saw tickling and got excited by it was in that movie "The Tuxedo" with Jackie Chan. Right towards the end of the movie with Jennifer Love Hewitt I believe, there is a scene where he has an insect trapped under a glass and he is sliding the shot glass looking thing all over her body. At one point he moves it up to her neck and her face scrunches up and she starts giggling. I immediately felt hot throughout my body and very strange. Believe this was also the first time I became aware of getting aroused and my own sexuality.
Don't remember what happened from there, but the first time I looked for tickling online, I found the Tickletheater, and the first picture I found was a picture of Beast Boy as an octopus in the training room tickling Raven's bare feet and she was laughing her head off. I couldn't believe there was so much tickling content in one place and I couldn't believe tickling was a fetish and other people were into something similar to me.