How to gain business skills?

Get a job.

Don't worry if it seems like a menial job. I've talked to people who needed work but didn't want to flip burgers or work as a cashier. I always recommend that, unless they have some valuable marketable skill, to take that entry level job. If you approach it the right way, it will be as valuable as a college degree. You'll learn world-class business systems and will work side-by-side with people up the chain of command. If you look at it as a positive situation and try to learn everything you can by soaking in everything around you, you'll learn business skills.

Personally, I was successful in business in my career. Looking back, the greatest management and business training I ever got was coaching youth sports at a young age level. You have a bunch of kids who don't know anything about the sport, and you have to teach them step by step while encouraging a positive attitude. You have to deal the the association board (upper management), and also have to deal with incredibly annoying helicopter parents. I applied more thing from my coaching experience than any other training I had.