This, pick something you really care about and would be willing to eat and breathe for a long, long time, because if you succeed at it, you're going to be doing it for a while. Don't aim for the Instagram style "exit for 1B in two years", if that's the goal the go to Vegas, it will save you a lot of time and tears.
If you don't, and if you're luckier than most, you might get stuck working on an idea or in an industry you really don't care about, just because you wanted to play startup and, like the poster above said, you read up on a bunch of success porn that's telling you that startups are the sexy, right thing for you.
Then you'll be stuck between a rock and a hard place, hating what you do for years, but not being able to leave because of all of the sunk cost, the shame of abandoning the team of people you've brought on board and convinced to chase a dream you don't really care about yourself, the reputation hit, the VCs who sunk all that cash into you, and the carrot of a potential, however tiny exit dangling in front of you. And the years keep adding up as you're chasing that carrot, still dreaming every day of getting out, but somehow that exit just never happens. If you're lucky/smart you'll find someone to replace you, but then they might kill the golden goose and you're still left with nothing, but at least you moved on, so that's a silver lining.
Fall in love with the problem and with the mission, that way, even if you fail or have a pretty rough time, at least you're working on something that truly matters to you. And "doing a startup" can't be the mission in itself, that's a recipe for disaster. It's one of those situations where even if you win, you still lose.