It may be something so simple that you aren't even thinking about it. Maybe the skill that you are going to use in your business is one you haven't even learned yet. You don't necessarily need to go to school either. You can learn many things on the Net for free. Unless you want to start a medical or law practice or other similar professional business, you likely won't need to have any specific degrees or credentials.
Entrepreneurs depend on their knowledge and experience.
Everyone has something that they are knowledgeable about. You may not think you are, but likely someone else does. It may be something that you would consider very normal, but there is likely someone out there that would be interested in the knowledge that you have. I can't tell you what kind of business to start. You need to figure that out on your own.