My advice is to contact
FastCap and see if they'll bite. If so, they pay you royalties and they do all the prototype, development and marketing. That's what they do. The owner Paul is a Porsche guy to boot. Click on the "ideas" tab.
If you want to have a garage business you'll need whatever licensing your state / city requires and a commerce friendly website like the ones Squarespace offers. You'll also need a federal tax id which can be your SS# if you're a sole proprietor. If your products have even the least probability for liability (bodily injury) I'd seriously consider setting it all up as a LLC. At minimum you'll want product liability insurance.
I've been in business for over 30 years in the woodworking business. I can say without a doubt it's a hustle. The FastCap route is by far the least pain for the most gain.