Here's the easy part, you should figure out a game idea and put it on paper, that's my first step in this journey.
In order to avoid any further problems I will say little about the game, instead I will put some of the key points of the things you should list in order to start the game:
1. What is the audience of your game? in my case I will create a game for kids between 4 and 5 years, why? because my boy has that age, and the games for kids are a little bit easy, catchy and does not required full 3D render in order to catch their attention, also I think the educational games are the best, if you're going work for free then why not give something to the world?
2. What is the game for? what is the idea of the game? here's the hard part, if you want to teach something (like I want) then you need to know how to teach that, here I will base the game on some other games that does something similar, I will create an idea based on some letters games I saw sometime ago and I will put some ideas together in order to create an original game.
3. The sketch: here you will put your idea on simple graphics, could be on a paper or in the computer, that does not matter, this will help you to realize if the game will be playable and your idea makes sense. I created some simple graphics using inkscape.
Ok, here's the first part, the next post will be about the articles you should read, the exercises you will need to face in order to start with your knowledge base.