If you know how to program, download Unity, Phaser, or Unreal and start studying "How to" tutorials (you can usually find full courses on Udemy at discount prices around $10.00).
If you don't know how to program, check out Construct 2, GameMaker, and ClickTeam Fusion. Then search for communities dedicated to building games with these tools.
Once you have chosen a simple, investigate "storyboarding" (for example, a quick search for "storyboard for games" turned up this link: https://www.cs.cornell.edu/courses/cs3152/2013sp/labs/design1/). This will teach you specifically how to go from a story in your head to a design you can build.
Once you have the tools and the process, try to storyboard and build a *simple* game. I mean *simple* like "Flappy Bird" or "Doodle Jump." Yes, there isn't much to storyboard, but at first, the thing that will limit you is your experience with the tools, not your ability to imagine awe-inspiring games. :)
Keep it simple, and keep at it. Making games is hard. There is a lot of technology to master before you can build the things you imagine. But making games is also a blast, and you will sometimes find that the "stupid little demo" you made is more fun than the magnum opus you've dreamed about since you were 6.
Go make something great!
Mark recommends the following next steps:
- Investigate Construct 2, GameMaker, and ClickTeam Fusion.
- Study up on "storyboarding for games".
- Design and build a small game.