If there was one piece of advice you could give to someone about to start their own business, what would it be?
Why are you starting a business? Be clear on your motivation. It's much more work and more stress than a 9-to-5 job, and in many cases you will make less money (not more). Be clear on your motivations or you won't stay the course.
Who is your customer persona and what *problem* of theirs are you solving? You have to be really clear on the problem. The problem should be urgent, pervasive, and you have verified that people will pay to solve it.
Have a lead customer (or several lead customers) that have already agreed to buy from you before you invest in any product development.
Be ready to do lots of direct sales. Picking up the phone and cold calling like a telemarketer, or going door-to-door like a salesman. Even huge companies like Stripe and Airbnb had to do this when they started. No amount of marketing can be a substitute for this.
Treat your customers like gold. Do whatever you can to give them a fantastic experience, and charge in return.
Don't quit your day job until you have some consistent customers and revenue.
Have a bias for action. Don't waste too much time thinking about what you might do, just do it. The way to get ahead is to get started.
Once you start, be in it for the long term. No matter how discouraged, keep trying new things every day. Think of everything you do for your business today as an investment that could pay off in 20-30 years.