Intro to level design is the tip of the iceberg, there is so much going on in the engine that you are best off defining the extents of your project before you go too much further so you can pick a learning path, i.e. are you doing 2D or 3D (VR?!), puzzler or shooter, etc. I think the next step after you have mapped out a basic plot is to decide how you want to present your tale.
From there you prolly want to look at systems, i.e. will you need an inventory, day night cycle, combat system, etc. Putting these into a point form list will let you know what you need to learn to achieve your goals. Some things will depend on other things like if you’re having inventory and armed combat you probably want to build the inventory system first with a view to it supporting your combat system.
Getting the underlying systems working is probably more important that getting the content blocked in - saves you from having to radically change stuff later, for example some of my climbing animations work with well certain heights and not at others, knowing that means that I can build props and structures to the correct sizes from the get-go and allows me to build a testing platform before the artwork becomes too involved to want to change (and change, and change). Once you have your systems in place you can design your levels in accordance with their limitations.
That said a good writer is like hens teeth. If this is your passion I would recommend starting to break your story down into story boards and getting some concept art made while looking for a team to help you put it all together. GLHF