How do I go about marketing a business?

I got an idea recently about a technology product I could develop for the construction industry. I got the idea while working with my brother plumbing when he complained he wanted to go paperless.

My idea is basically small printers mounted in the vans that will interface with a mounted laptop or smart phone. The application I will develop will handle all the printing, gathering of customer data, storing notes on customers, process credit cards, storing invoices, and even GPS tracking\vehicle monitoring (Not sure if I want to implement this. You could monitor if your employees are abusing the company vans and\or are not where they are supposed to be).

Ok I have an idea. I can develop it. How do I market it? My plan was just to drive to all the local plumbing, construction, and AC companies and demonstrate my product. Is this a good strategy, or are there better ways to market? I just don’t want to waste my time developing something that can’t sell.