Selling shares of your company is really dependent on the laws and regulations of the country you reside in.
If your product/service is really popular then you shouldn't have much difficulty finding investors. When you sell shares that's what you are basically doing, finding investors.
If you want to place your company's shares in the Public market, then you'll have to look at the laws and rules of your country. There are also different types of exchanges. Each exchanges also has it's own rules and laws.
Your best bet would be to find legal representation to help you through this process.