I was in a similar position to you. In fact, you're ahead of me as all I had was a single image in my head of the end of the story. So, although the guys here are right in what they say, don't also be discouraged.
All I can really do is say what I did, which may or may not be any help.
Firstly, map out the story/game you want to tell. It doesn't have to be all the dialogue but it does need plot beats and key points. This will allow you to see how long it will be, what art assets you need, what game engine works best.
Once you've narrowed down those choices play around with the engine and/or art software for a bit. Just see what it, and you, can do. Learn what you need for your game.
Then, and only then, would I suggest making the game itself. But even then you need to think about the characters in your game. What do they like, how do they react etc?
As a point of reference it took me about six months from that initial image to my first release. But I am a bit of a ditz. Your mileage may vary