Look around you and come up with a product that will solve a problem. Most of the world billionaires became inventors and registered their prototype with a trademark/copyright board.
Alternatively, you can sell your prototype to a company with enough capacity to produce for the mass market. You can have an agreement on how you will share any revenue that will accrue from the sale of the merchandise. This ensures a guaranteed source of royalties for as long as your prototype is in use.
If you’ve never created a product then checkout Quirky.com where you can get funding and build your invention.