I don't remember my first time writing fanfic (or reading it, for that matter). I have to assume it was Star Wars. In junior high, I wrote this amazing (read: mega cringe) story where Luke Skywalker crash-lands on a sparsely populated planet. He is, of course, injured and therefore must be cared for by a local farmer and her beautiful daughter, Antikee. Antikee was my own alter ego (*shocking*). And if you knew anything about 13-year-old Alison, you can rightfully surmise that Antikee and Luke totally boinked.
Sadly I don't have that story anymore. I do, however, have a nice collection of my teenage fanfic—mostly X-Files stories. There is a super-painful-to-read X-Files/Indiana Jones cross-over with this paragraph in which I demonstrate just how much I sucked at spelling when I was young (I still suck, but not quite this bad).
Now Indiana proseeded with caution. He looked where her stepped and was careful not to snagg any wires. Without furthur mishapes he made it to the chamber he had been seeking. Inside this cavern sat a small statue on a stone pedastool. The stature had been hand carved in pipestone. It was the last known piece of it’s kind that had not been taken illiaglly and sold to “collectors” for profit. It was the kind of thing that belonged in a museum.I never finished that particular story (I like to think that's why it's in such dire need of proofreading). None of it had seen the light of day until now. But I did finish and share many a fanfic. I even published several stories online to fanfic pages long since lost in time. Apparently this is very common among writers.