What should you know before starting a company?
Startups are challenging both physically and emotionally and it is important to learn not to freak out. The most challenging situation for you might end up creating a strong company, but that won't become apparent till after the fact.
To survive entrepreneurship, founders have to understand that emotional roller-coaster is coming, expect it, and work through it. After all, startups are limited liability companies for a reason, and even if the business fails, everything else will be okay.