Here's a list of places to post your writing online. Most of these are fan fiction sites. There are many Harry Potter fan fiction sites—I’ve only included one. To find more, just google Harry Potter fan fiction. This is true for soap opera fan fiction, vampire fan fiction, etc. If you don’t think fan fiction sites are worth your bother, consider the origin of Fifty Shades of Gray.
I am in no way endorsing Fifty Shades of Gray. I haven’t read the book. And I never will, which means that I’ll never make a judgment call—although I guess I am by my abstention. And I’m not endorsing fan fiction—I’m just saying great big, take your breath away things can happen by gaining an online readership.