If you have an idea for a product, the first thing to do is to see if such a product already exists. Let's say for example, you had an idea for a bread bag tag that could revolutionize the industry. After searching online, you couldn't find a product that had the same level of detail, or value, as that which is currently on the market.
If you're ready to prototype it, you don't have to go out and buy a 3d printer yourself, you can go from idea to prototype from the comfort of your couch.
First, do a search on Thingverse and Pirate Bay to see if someone else has designed something that is similar. You do a search for bread bag clip and find this prototype. It's missing a key feature that your idea has, so you're ready to design your improved version.
Download SketchUp and start your design from scratch, or import the .STL file you downloaded from the Thingverse prototype link you found earlier, and modify it in SketchUp from there.
Once you're happy with your drawing and are ready to have it 3d printed, post it to 3d hubs which is a crowd sourced website of over 10,000 3d printers. Once you enter your drawing and location, someone in your area will connect with you about printing your prototype design for you, and you can arrange pickup/delivery.
It's never been easier to go from idea, to prototype in your hand, thanks to open source, and 3d printers.