Hi!
I think one major mistake beginner writers often make is writing just for the sake of writing. What I mean by that is, there’s this idea that the more words you use, the more meaningful your writing will seem — but in reality, less is often more when it comes to getting your point across clearly.
Another common mistake is starting without a clear plan. I really believe it helps to first figure out the main idea you want to communicate, and then think about the best ways to express it. That way, you avoid adding extra points or ideas that might take away from your main message.