Some people have a hobby that they really enjoy doing. It may give them a lot of pleasure and often this leads people in to thinking if they can make a career out of it. With all hobbies, somewhere along the line someone is making money from it but you need to spend some time doing some research to see how it all works and to carry out some market research.
If for example your hobby is gardening, then you can most certainly make a career out of it. Self employed gardeners may not make a huge amount of money and often the work is more seasonal so you may have to bear this in mind. Have a look around and see what other people are charging for their services. See if you could match or beat their price and ideally if you could add anything they do not offer to give you that unique selling point.
If you are going to have to invest a fair bit of money in to setting up a business, then even more thought may need to go in to it to ensure that you are not going to waste all of your money.