First of all you need to decide which platform you want to develop for. To make a game to earn money, isn't always the best approach if it isn't your full time job. Make a game because you want to, and because you have a great idea. There are plenty of great games for free, so why buy yours? Maybe in app payments, and ads are better? As you see there are several decisions to take.
But if you want to make a mobile game, you can distribute them through Google Play, Apple Store etc.