sometimes I have an idea or scenario that I think would work for a game, sometimes I find a game or style of game I like then try to make my own. Sometimes I think about what do I love about a particular type of game and then try to improve on it. Inspiration can come from anywhere, the trick is to be ready to experiment and try it out. Keep a notepad, etc handy for when you have an idea.
Another great source is the how do I section on the site here, I look at the issues others have, then try to solve them and often in the process get inspired to do my own thing.
Also I like to create stories around things, so when I think of a story, I think what type of game that might work with, and start there...
Or the classic, you play a game, find something that frustrates you about it and then decide you can do it better.
Also, realize that not everyone is "feeding the family" with apps... for every good money maker out there, there are a million failures. Don't get your hopes too high on your first game being a success. Like any craft it takes time and effort. Gameplay, polish and detail are needed if you actually want to stand out in the overwhelming sea of mediocre crap that is being published every day. Good luck and welcome to the modern day land rush!