This is assuming you already have a physical product created or are planning to create your own unique product. If you're simply reselling another product the answer would be slightly different.
Validate your assumptions by getting your product into a potential buyers hands as soon as possible. Any product feedback you can gain early on will save you massive amounts of time and money in production as changes come about.
If you can do small batches - this might sound counter intuitive and be slightly more expensive but produce a small batch and get it into a market like Amazon or a market relevant to your audience. Having this small batch will allow you to gain invaluable product packaging feedback, product variation and size/quantity feedback, etc. All of that learning you can then apply to a larger batch and not be stuck with lots of unwanted packing, sizes, or quantities.
Determine your growth engine and maximize it to get direct feedback from your customers early on.
Spend time with your initial customers listening to what they are really saying and do not be afraid to make a pivot - it's more important to solve the customers real problem than to launch your own good idea - the former is something you can build a business on.
Look to make relationships with industry influencers and leaders in your segment - doesn't always work but if you can get an influencer to support your product that is an excellent way to gain early traction. Before asking them to partner or recommend your product be sure to get their industry feedback and perspective it might be some of the best advice you get.