To build something successfully, there needs to be a certain motivation. This motivation comes from your goal. The question to ask is why do I want to build something?

1. Are you building something for fun? Look at what you do for fun. Narrow down on these topics and then decide to build something. For e.g. spend a lot of time on HN? Build an HN Chrome/Firefox add-on that does things you want in HN.

2. Are you building something for learning a new technology? Narrow down on the technology that you want to learn about. Now think of apps to build using that technology.

3. Are you building something for additional revenue? Find pain points in your industry and create apps to solve them.

Without the right amount of motivation, even if you had started with any of the projects mentioned above, it is very likely that you would not be able to follow it through.