The most common advice people seem to give to beginning writers is to "write what you know". I have a lot of opinions about that. I'll be touching on that soon, but what I want to cover here is, what I think, a slightly more important piece of information. Know what you're writing.
It sounds easy, right? Like, how could you not know what you're writing. Maybe if you're an outliner and a planner, you've already got it all figured out. That's not me. I can barely plan out a snack, let alone a story or an entire book. I tend to start with a character or an idea and kind of just go, hoping I'll eventually stumble across a plot. So, often, I have no idea what I want to say with a piece until I'm well into it.
Which is fine, by the way. I'm not saying you have to know from the get-go. But you should definitely figure it out before you let anyone talk you out of anything.
Example time! I just finished reading Neil Gaiman's The Graveyard Book. I found myself, at one point, wondering exactly what the plot of it was. We started with the main character, Bod, as a toddler and it's just about his life. His normal(ish, I mean, the kid lives in a graveyard and is raised by ghosts, so as normal as you can get with that), day-to-day activities. I thought maybe it was meant to be a series of connected short stories and I just missed that part of the description (because I honestly didn't really read the description, I just saw his name and the title of the book and went for it). By the end, it was clear how it all tied together. It was pretty great.
But, if someone had brought a story like that to our writing group, I would have said, "This is really cool, but does it further the plot?" And, if the writer had listened to me, a great story might have been lost.
Christine Taylor-Butler said that you should never take a piece to be critiqued or edited until you know what it is you have. So, that's really what I guess I'm getting at here. You don't have to know right now or tomorrow or even next week. But you should know before you let others pick it apart and before you change it. Let it turn it into whatever it's going to be first. Otherwise, you could miss out on something awesome.
But if you get there, if you figure what kind of animal you've got and you realize that it really doesn't need that vestigial tail, cut that thing off. There's no shame in that.