Cofounder relationships are among the most important in the entire company.
You have to find a small market in which you can get a monopoly and then quickly expand.
You need conviction in your own beliefs, and the willingness to ignore others naysaying.
If you can just learn to think about the market first, you will have a big leg up on most people starting startups.
There are exceptions of course, but most companies start with a great idea - not a pivot.
Mediocre founders spend a lot of time talking about grand plans, but they never quite make a decision.