Completely honestly it wasn’t even anything online. We had just gotten a computer and I was trying to use it to connect to a BBS for the first time. The modem volume was up all the way and the sounds startled the ever loving shit out of me. I was afraid I had broken the computer and was going to be in big trouble so I panicked and unplugged the computer from the wall, and then died laughing as I realized that computers probably don’t make noises like that when they break.
As for content online it was either rotten.com or nifty.com, but not sure which one was first