It's not uncommon for people to learn how to sharpen knives, become filled with pride an enthusiasm, then get dollar signs in their eyes and start thinking "I'm really good. People will pay me to do this". But there's a big difference between being able to sharpen a knife, and operating a successful sharpening business. And starting a business when you don't know anything about that business is a great way to go OUT of business.
My advice, if you want to start a sharpening business, learn the business from the inside. Find a sharpening business and ask for a job. If you're THAT good, then perhaps they will hire you. And if they can't afford to hire you, then volunteer your time in exchange for learning the business. This is how many successful business owners got started. It won't immediately fill your pockets with money, and you might end up doing a lot of schlep work, but the knowledge and experience you gain might prove invaluable.