crazy7 that's a good question! Although I've never started a startup, I'm definitely one of those people who has an endless "million dollar idea" list on my phone. I watch Shark Tank and Dragon's Den religiously and love to read books about new companies and be wowed!
From my reading and watching, it's not likely you'll be able to sit around and come up with a startup idea. Usually, they start from a problem. It's also a problem lots of people have (big market) and it doesn't have an easy or accessible solution.
Whether you start with yourself, your community, or another community, you need to identify why someone would need or think they need your product or service! I love to think about Tinder and Bumble as as example. Bumble is almost a blind copy of Tinder, but forces women to message men first solving an extremely common problem. Boom, $250 million dollar company!