You! It doesn't matter how young or old you are, or how well you write. Just begin with what you know and write about it.
Golly! You can write pretty much about anything. Your sister or brother. Your pet. The dust under your bed. See what I mean??? Anything. I like to write about positive things that make people think, smile and generally feel good.
Read what you've written. Maybe to your best friend, a parent or your pet salamander. Sharing your story with kind listeners is helpful.