The "idea for a startup" is just a place to start. Pick one that you are passionate about, because you'll need that to sustain you. Implement some (tiny) part

The "idea for a startup" is just a place to start. Pick one that you are passionate about, because you'll need that to sustain you. Implement some (tiny) part of that idea; show it to some customers. Find out it's not that great an idea. Learn more about the customers in the process. Think about what their real problems are and come up with new ideas to solve those problems, or maybe think about other customer groups that would get more value out of (would pay more for) your idea and tweak it to fit them. Follow those new ideas. Repeat until you've found an idea that's sustainable (big/profitable enough market etc).