I'm trying to describe a dark and quiet forest for a book. I'm stumped. How should I describe it?

You need to read novels. Lots of novels. Any novels. Notice that they all have perfect spelling, perfect punctuation, perfect composition. Notice where they use one space and where they use two. Notice that every speaker in a dialog gets a new paragraph, and every paragraph is preceded by a blank line. Notice any writing techniques or figures of speech. Take notes so you know where you found what you know. It takes a lot of reading to be a writer.

Don't go looking for cutesy lines to use in a story you are writing. The result makes it is obvious that you did that. Just tell your story whatever way you are able. I have a file of 850 lines that I have saved for thirty years. I have used five of them so far.

Go online and look up pictures of the Hoia Baciu forest in Romania. Then write about how it makes you feel

Look up "Suicide Forest". It is very well described; you can almost see it.