Hey there Indigo 
No need to apologise, these are the exact kinds of discussions that people should be bringing to the forum, and I hope that by having them you will have some ideas for how to proceed with your writing.
So I’m not familiar with what story you want to write or how this fantasy forest world would fit into it, but I’m going to go forward initially here just assuming that you want this forest world to be a setting that features in your story.
Before anything else, I think my first piece of advice for any and all questions like this is going to be: just sit down and write something. This sounds dumb and unhelpful on the surface but hear me out. There are a couple of ways to apply this advice in a useful way.Just write stuff 1: Generally...
Firstly, as a writer, whenever you have an idea about something to write, you should write it down. Ideally, rather than just writing the idea down (which is often counter-productive I find...), you should write an actual piece of story that uses the idea. This could be a paragraph, a scene, a full story-board or a full chapter etc. Just write something that gives an end product. The reason to do this is to give yourself something to build around and a collection of material that you may be able to pull together into a more cohesive whole at some point in the future. What you might find is that you’ve written something that at the time seemed random, but, now that you are trying to build out a specific place in your story, you already have a short piece of writing that can fit in to your narrative with just a few minor tweaks, and the concepts explored in that short piece can be integrated into the world-building of the new setting presented in your story.
Just write stuff 2: In your setting...
Secondly, when you have a vague setting in mind, write a bunch of random short stories based in the setting that have nothing to do with the main story you want to tell. This is some world-building advice from YouTuber Daniel Greene that I have found that I agree with. It’s like discovery writing for world-building:
So you’ve got nothing at the start apart from a “fantasy forest world”, sit down one night and write the first scenes and dialogue that comes to mind in that forest. Suddenly, out the other side, you know that there are tribes of Fey creatures that live in the forest, and that a war between the tribes has driven one tribe underground. If you are really good about it, you know the “hero” from the “victors” who scored the blow to turn the tide of battle and what that was, and you know the “hero” who saved their people by finding a safe haven and distracted the enemy whilst their comrades escaped. Next session you find out that a council of Fey elders has been told by the researcher Artemis that magic is beginning to fade from the forest, and they decide to suppress this knowledge. Also the method of testing is by tapping the trees and examining their sap, so you are establishing that their is fey magic held within the trees. In one session you find yourself writing about a young hopeful Faerie who makes a pact with a dark being in a desperate attempt to restore magic to the lands, resulting in the seeds of blight beginning to spread across the forest.
Write as many of these short stories as you can because you can then have an amazing advantage in your world-building and for your story-writing. For one, all of these little stories can build into the current picture of the setting: You have a Fey empire on the over world, and a reclusive Fey enclave hidden underground. The empire fell with the fading of magic and a civil war, known as the Artemis War, but the enclave remain safe whilst they tap into and protect a primal version of the magic. All the trees on the surface are hollow and blighted, but wonderful plants are able to grow underground. Dark, twisted creatures roam the forest and the blight spreads across the world, consuming Fey that hunger for their lost magics. See how everything kinda leads into the next thing? Moreover, when it comes time to write your actual story, you can present your world-building in incredibly deep ways without the need for exposition. For instance, your characters come across clusters of trees all with tapping tubes in them, then later discover Artemis’ alchemy workshop containing magical tree sap and a collection of old letters rejected by the council of elders. If the readers have been told previously that there was a war named after Artemis, then they should be able to put all of this together to work out a general idea of what happened. And even if they can’t, the idea that there are puzzle pieces that they are finding at all hints to them that there is a deeper story there. Just finding a sword that has a name “Evergreen”, which belonged to the hero “Caelen’dar” and is said to open a hidden gateway to underground magics speaks volumes if the characters never even go near the hidden Fey enclave in the story at all.
These are methods of essentially writing stuff first, and working backwards from the product to world-build. It is rewarding but time and effort-intensive. If you are more inclined to plan before you do any writing then I have a few small bits of advice for that:
Find an interesting feature for your world, then ask questions and find answers
A different angle to take is one that I heard Brandon Sanderson took when developing the Stormlight Archive (it was in a chat or interview somewhere on YouTube but I can’t remember which one). From what I remember, he had the idea for a world where a huge storm would regularly sweep across the land (I can’t quite remember if this came first, justifying the crustacean-like creatures, or if the creatures came first, requiring the storms...). He then asked himself, ok what causes these storms? So now Roshar has 3 moons in an unnatural orbit of the planet, which potentially cause these colossal storms of water and lightning. Then he asked himself, ok so what does a land scoured by such storms look like? What would plants and animals need to have to survive such conditions? And such we get some creative but realistically adapted flora and fauna as outlined in the first 7 minutes of this video here. So the take home technique here? Think of a unique phenomenon for your world/setting, and then ask questions about how that phenomenon is caused and the effects that phenomenon has on the setting. Does it happen all the time, irregularly or in a cycle? What things can live with the phenomenon? Does it need to be adapted to? Can it be used by some beings? Does it affect where things live? Basically choose a selective pressure for your setting, and then see where natural selection takes you! Evolution of your setting! (Btw, I haven’t watched it yet, but “Alien Worlds” on Netflix might give some inspiration for this kind of world building). If you have cool ideas for experiences, try to fit them into your setting
Try to think really hard of the kind of experiences you want to have in your story, and then work those in to your setting as best you can. For instance, I really like the idea of a whole nomadic city of merchants in the desert. Where the entire city is tents and huge pack animals, and it is constantly moving to find trade. So if I ever write a story with a large desert in it, you can be sure to see that in there. What kind of experiences can you think of that just really fit into a forest setting? A city of tree-houses? A witches' coven? Just find a collection of really cool ideas that would work in your setting and fit them in there as dots that you can connect after-the fact. (This is a bit like a less in depth version of the short stories from before.) And remember that not all dots need to be connected. Like in the real world, some major events are distinct from one another, resulting from completely distinct chains of causality, so you can connect your own dots in clusters that make sense instead of trying to force one big picture of the world where everything is linked.
Show the tip of the ice-berg, pretend that it spans deep below
Remember, you only need to world-build just enough to provide a foundation for your storytelling; it does not actually need to be a whole cohesive history and geography. The analogy that is often used is an iceberg floating in water. Your story is the water level, and it is punctuated by these peaks of interesting things from your world (the tip of the ice-berg showing above the water). These peaks should hint at the existence of something that spans deep below the water level (by eluding to history, geography, theology, names, prophecies), and maybe you can get a blurry outline of what lies immediately below (general/vague explanations of that list), but the readers don't need to know anything for certain outside what is necessary to tell the story you want to tell. So don't worry too much if you can't get every dot in your world building to perfectly connect or you don't know how to flesh out absolutely everything to make it all 100% engaging and interesting. So you don't know how to get a piece of geography that the characters are seeing to make sense? Who cares? Just don't explain it or be vague about it.
You might have guessed that I spend a lot of time listening to writing advice, author interviews and writing lecture series on YouTube... I promise it is quite helpful for stuff like this. (I even find the D&D podcast I listen to to help a lot...)
The stuff I outline here is more advice for how to approach world-building yourself. Sorry it's so long... If you are looking to spitball ideas for your setting then we can do that too and see what sticks. Sometimes it helps to have others to bounce ideas around with.