I tried to make one of those before Ring of Fire and it was such a big job that I quickly gave up due to the complexities of building a convincing forest. I used Quixels megascans to populate the scene on my ArtStation but nowadays I'd grab a couple pre-existing forest packs from the unity asset store to get started, and cultivate your own rendering style, then teach yourself to make your own foliage to build hero-pieces.
If this is your first game I would recommend starting on small indoor spaces like we have here in Ring of Fire, you can grab a bunch of free assets online and combine them relatively quickly or buy an existing scene.
https://twitter.com/animtree/status/1143230674100510728?s=19
The reason you probably haven't gotten around to it is the idea is too big, pick something smaller first that you don't care deeply about, and make that as a way to learn your tools.
We use Unity as our rendering engine, you can combine it with any number of plugins such as adventure game toolkit + fungus, ink or yarn as your narrative engine. It's not important what 3d package you use but I would recommend Blender 2.8. I use Photoshop for textures, concepts, and store art, but there are free alternatives like Krita.
I'd love to know how you get on, feel free to post about it in our community discord, happy to give you feedback and support!
Chard.