Brent's YouTube video is spot on. Culture is defined as the beliefs that the organization has, which if tested, result in a visceral reaction on the part of most employees - similar to the monkeys in the cage.
You as the founder of the company are owner of the culture. So, what belief system do you have that is being imprinted onto the company's? What employee behaviors do you encourage and discourage? Is everyone suppose to read your mind....that is likely to be a recipe for disaster. Is it acceptable to get into shouting matches and physical fights in your company? Don't laugh - that was part of Cisco's culture before John Morgridge became CEO. Another example, which I do not recommend, and goes against my own beliefs - try interviewing at major defense contractors such as Lockheed Martin proclaiming that there is a need for a new World order that demands the U.S. become a socialist-communist state - you would likely be asked to leave before the first interview was over. Why? You would not fit in the corporate culture, because your belief system does not match the corporate belief system.
If you do not overtly state what your company's culture is, then your own unstated belief system is what is the de facto corporate culture is. What a cheap way to find it out? Spend one hour writing out what you believe in...hard work, long hours, no lunch breaks, company over personal life, execution over creativity, pets not allowed at work, children are a distraction to the real goals, Python coding over ruby-on-rails, great sales people over great coders...these are examples. Be brutally honest with these...these are not beliefs you want in the company, these are beliefs you have personally. Then ask two of your employees who you trust to review the list and ask for them if they agree/disagree/add - again, what is, not what they think it should be...the ones you three agree on are the foundation of your current corporate culture.
If that exercise results in you and the team realizing that your belief system has a few beliefs which might stifle growth (I.e. - won't attract great coders or won't attract great salespeople are two extremely common ones), then you have a decision to make...change the belief or accept the consequences...Your choice.
If you need help with this, look me up on LinkedIn and contact me personally at firstname.lastname@example.org.