The best way to have a good idea is to have lots of ideas.
While observing successful people, you will notice soon enough that they have some things in common. One of those things is that they all have a lot of projects going on. Their minds are infinite pools of ideas and they have the courage (and the means) to try their ideas out.
No matter what you want to achieve, having a lot of ideas in your brain is an awesome way to start! Yes, there may be some stupid ones that you will rule out, but if there is one good idea in a hundred attempts, then you better start dreaming and opening your mind towards new concepts and inventions.
Allow yourself to be creative and to dream, soon enough you will be rewarded!