Maybe we can sneak up on a good answer to your "one/best thing" by pointing out the one/main thing you definitely, positively should NOT do:
-Don't go out there and start selling anything!
You want people to line up to buy what you have and you don't accomplish that by going out and making countless desperate sales pitches.
That's because, no one likes to be interrupted by salespeople and their self-serving sales pitches.
Instead, build an inexpensive prototype of your product or service and go show it to your suspected customers. Tell them what you're trying to do and ask for their opinion. Then listen. Take notes.
This process will clarify a lot of things for you. Are these the right customers? Is there a real demand—do they want to buy? Does your positioning need to be changed? How can the product be improved? And much more. And you accomplish that by positioning yourself as a leader and innovator—and not as a salesperson.
Once you have a real grasp of who your customer is, what they need and what they're really willing to buy, you should start to position yourself as an authority by staking a leadership position in organizations or events where your customers congregate. By associating with other trusted brands your own brand benefits from a "halo effect" which gives you instant credibility and trust and allows you to easily break down the usual resistance. For example, giving a chamber of commerce talk to a roomful of prospects positions you as a leader with credibility. If your talk is good, guess what happens? People line up to ask YOU how they can buy.
Obviously, there's a lot of pre-work that needs to be done within those organizations to establish trust and credibility so the sooner you start, the better.
Lastly, be patient. Stay persistent. It takes time to do all this work. Getting clients works like a bank account. Don't seek to take out what you haven't put in.
I hope I've been helpful. Please let me know if I can be of further assistance. Best of luck.
