@KatAnimated89 uhh, I don't know if you still need help or not, but, I have a few ideas:
(1) Maybe, instead of doing the normal "I'm a serial killer," you should maybe do something like "I'm a stalker," or something else a bit creepy.
(2) Instead of a sad backstory, try a happy story. For example, "all of my family members are still alive, but then someone came along (or someone did something) and now. . ."
(3) Instead of making a person, maybe try making an animal or creature instead. The Rake is a good example of a creature creepypasta.
(4) Try to get basics of what you want them to look, act, and do. But don't go to overboard with abilites, and make sure to balance them out before you put anything out there.
I hope that kind of helps. Again, these are just suggestions to try and mold something.