OK, first you have to decide on what structure your business will be. If you are working with someone else, perhaps a S corp would suit your needs.

Next, you will need a local business license.

In general:
1. Find a business lawyer and a business CPA (Accountant), have a meeting with your partner, and them, and figure out how you want to structure your business, what the incorporation costs are going to be, etc. ~$500 is a good number for legal fees & incorp. costs, but that varies a lot by state. I recommend the laywer & CPA be in the same room: they usually have different perspectives on things, and so it’s best to get them together.

2. Incorporate—In the US you may end up as an S Corp, or an LLC—LLC’s have changed a lot over the past several decades. Your lawyer will also help with any licenses you may need—that varies a lot by state and locale.

3. Get Advice. Your local business community wants to help you. TiE Global has lots of local chapters, and they’re awesome: https://www.tie.org/ But Rotary Clubs, local business support NFPs, etc. all help.

More specifically:
A. Don’t talk about your product on the internet just yet, but don’t be afraid to talk to people in person. Ideas are cheap, implementation is hard. Chances are someone has had your idea, your goal is to make it happen better than they can. If it’s patentable, you have 1 year from when you announce the product (or “make public”) to file patents, before you lose the ability to. (See “Ideas are cheap”—you may not need or want to go through a patent process—typically patents are $5k to $15k to get.)

B. If you’re developing a consumer product, realize that you’re going to need to have or raise a bit of capital. There’s a lot of testing, certifications, and spin-up time that goes into consumer products, which costs money.

C. Don’t be afraid of marketing. If you don’t really know what you’re doing when it comes to marketing, hire someone who does.

D. Generally avoid hiring family and friends. If you have to let them go for whatever reason (poor performance, bad fit, or downsizing), they’re really hard to fire. If they’re critical to the company (my wife is my CFO), it’s fine. Hiring friends and family for “seasonal” or “overhire” positions is also ok—it’s easy to get rid of them at the end, and they haven’t built their lives around the job.

+1 on hiring an attorney. As an attorney who handles commercial litigation, I cannot tell you how often people come into my office who start a business with a friend and end up in litigation over the business because it was not set up properly (i.e., they put too little in writing because they “agreed” on everything and didn’t need to).

Spending a little money on an attorney now will likely save you a small fortune at some point in the future when the business becomes successful.

Look up SCORE in your area…..a group of retired and active successful business consultants…IT IS FREE….they have on-going workshops etc…..and know that you can do all the planning in the world…but you have to have a great idea, determination, resilience, fearless, be willing and patient. Good luck

I would write up a business plan before doing any of the above suggestions (which I do agree with). Figure out what it’s going to cost to produce the units, think about what people will be willing to pay for each unit, are people going to want to pay that price (look at similar products and what the market will pay). Figure out how many you will need to sell to cover your business expenses, then figure out how many more you’ll need to sell so your friend and you will generate enough revenue to make it worth while.

Estimate your expenses (legal, marketing, insurances, accounting, etc.) and always assume things will be more expensive and take longer than you think they will. Get feedback from lots of people (particularly people who will be using your product). Learn how to make several small prototypes out of cardboard, with make-believe magic-marker buttons etc. Test it, make changes before building the thing out of the final expensive materials. Re-test and make more changes, etc. Then build a functional prototype, test with that.

Focus on design. This is perhaps my all-time favorite quote of Steve Jobs:

“In most people’s vocabularies, design means veneer. It’s interior decorating. It’s the fabric of the curtains of the sofa. But to me, nothing could be further from the meaning of design. Design is the fundamental soul of a human-made creation that ends up expressing itself in successive outer layers of the product or service. When you’re a carpenter making a beautiful chest of drawers, you’re not going to use a piece of plywood on the back, even though it faces the wall and nobody will ever see it. You’ll know it’s there, so you’re going to use a beautiful piece of wood on the back. For you to sleep well at night, the aesthetic, the quality, has to be carried all the way through.”

In other words, design is all about putting yourself in the user’s perspective and making the entire experience as delightful as possible. If you always focus on this last point and you make something really great, you won’t have to push as hard and your marketing efforts will be much easier, your customers will be happier and will drive your sales for you.

Best of luck.

You might mention the nature of the business (not specifics): Material costs? How capital-intensive? How much financing do you need? What kind of financing will you use? Any idea what your breakeven point is? Will you need loans?

All these things somewhat affect how you’ll get started.

I second what @gorillapaws said! (great advise!) After four startups, I can honestly say, it’s the best thing you can do. It will help you focus and ask the right questions regarding its’ viability. Best of luck to you!