As other people have posted here, do small sections at a time.
I also suggest you try and think of some small achievable projects to break it down for you.
Similar to you, I got a idea for a game that I want to make. There are heaps and heaps of skills though I need to learn though. For the last 4 months I’ve dabbled in a bit of everything, completing a number of UE4 tutorials, watching the twitch streams etc, trying to find something to focus on.
I eventually discovered some nice 2D assets and I’m using them to create a little 2D platformer. Just so you know, my game idea is no way shape or form a platformer. However by having a fairly complete set of assets that I can use to make a fairly nice little project it allows me to focus of gameplay, and so I can just learn the engine without worrying about art assets yet.
In terms of getting your creativity juices flowing for your game idea, I suggest you do a lot of reading - historical fiction/non-fiction, documentaries etc. It will give you ideas for heaps of things: game ideas, environments, story lines, props, characters. I’ve got a notebook which I log of lot of random ideas that come into my head. So though I’m currently working on other projects, I still have my eye on my ultimate project.
Best of luck with it. These things take time.