I have done this into enterprise but I would say this advice also follows for B2C. It is hard, slow but very interesting. I would suggest a couple of things: (1) there are so many “experts” on social media to follow and get up to speed. Use this to build a casual network, ask questions on the group boards on LinkedIn, and make LI connections but ask for calls for 20 minutes to pick their brains. (2) I would partner with one or two people who can fill in the gaps in terms of expertise. You can tripped up by industry norms that you don’t know about and make no sense; obscure regulation and jargon; silent influencers and just a general understanding of how an industry moves in terms of adoption etc.

Having done this into a highly technical "expert mindset" industry, which was mature, I ending up looking for a partner who could fill in the gaps. I found someone on LinkedIn that became my partner in this and was extremely helpful once I got in front of sales prospects in terms of technical language, regulatory information and experience in implementing technical solutions. By the way, as I find with all early stage sales, the things that tripped us up were not industry knowledge related but as always, how much energy your champion could into it; how much political capital they had to push stuff through and whether they’d been successful before doing this. Happy to talk to you more about my experience if that’s any help.

Partner with someone that you trust who has a business and start up background, preferably in the industry you want to start your company in.

The knee jerk reaction to this question could be, "Why would you want to do something you know nothing about?"
Perhaps, you like the trade? Perhaps, you see the value? Or just maybe, you see something else that intrigues you! The best part of the answer here is that no matter what, you will need to challenge yourself physically, emotionally, and financially.
As others have said, the availability of online training is phenomenal. You could gleam tons of knowledge, or even just enough to be dangerous! Once you think you have figured out the space, work at it for 1 more hour.
That said, how do you think a kid out of high school becomes an electrician? Starting as an apprentice, the student will become well versed in both text book theory and boots on the ground theory.
We work with a number of beginners in the field of finance, grant writing and real estate. We are always happy to schedule a call to discuss your needs and point you in the right direction!

This will totally depend on the sector. You can buy a small business in the sector and make them bigger. You can consider becoming a Franchise operator. if there are franchise available. You can hire people who already work in the sector and get them to work for you or partner with you. Just get as much knowledge as you can about the sector.

I would suggest u read some books and watch a couple of YouTube videos in your chosen niche to get an idea of what it is to be in that industry or probably getting skills through the books and free videos.
After that u can choose to help someone grow his business like am doing currently or start a business from scratch to get enough experience.
But,u don't need all the experience stuff to know an industry first, u just need to learn about it and be willing to learn. That is all!
For example,I am learning all I can about ecommerce through books and interacting with store owners and giving tips which helps them even though I don't own a store now. So just be willing to learn and learn.