Cant stress this enough. Start with the simplest possible game project to test out the waters! You will be to naive and want to tackle big things but you will probably regret that. Theres alot to be learned by starting small and then create something bigger once you have some experience with the market.
In my own experience, it is not only about starting simple, but also finishing it simple. I know you're not saying it isn't, but what I want to emphasize is that it is too easy to say that you're done with something simple and then move on to the next/bigger thing. For example, what eru suggested, if you build a Tetris-like game. You probably can have the gameplay finished in a few hours. And then, move on? Or spend another week on polishing, menu's, sound, graphics, copy, etc. I think the most valuable part of the lesson of starting simple is in finishing it. (And they don't tell you that.)
This is probably the best hidden advice on this page. Build complete projects. Make it a habit. If you just make gameplay prototypes, that will become your habit and you'll struggle to ship your big idea when the time comes. You'll be missing the final 90%. You won't know how to prep game art for example.
This applies to all creative "package" projects. Learn to ship.
If you just think in terms of finish a project, and not so much the market, Tetris is a good start. And shouldn't take too much of your time. (And if it does take lots of time for you, you are not ready for bigger things.)
Cant stress this enough. Start with the simplest possible game project to test out the waters! You will be to naive and want to tackle big things but you will probably regret that. Theres alot to be learned by starting small and then create something bigger once you have some experience with the market.