The first thing you need to do is come up with a concept. This can be very general, but then you can start getting down to specifics. Everything can be broken down into smaller pieces, and once you have the smallest piece, you can start designing the game mechanics.
For example consider a game concept called "Snowball Fight", with the idea that players would create an army of neighborhood children to engage in wars with other neighborhoods in epic snowball fight battles. There would need to be the collection of resources (snowballs, snowbricks for snowforts, wood for shields), purchasing of better snowsuits (armor), children recruited to be designated as workers or fighters, a build phase and then, of course, a war phase. This is all pretty general, and you need to start thinking about specifics. What stats will the children have? Strength? Speed? Health? How will these be incorporated into determining if a hit is made, or how much damage is done, who goes first? Or will all combat results be determine simultaneously?
These are the kinds of questions that you will have to consider when you are designing your game.