That is too open ended a question to answer as you asked it. It is safe to say that 100% of the time a successful business starts as a passion of the owner and that his product or service provides something that someone wants, further it does it better or cheaper or for the first time. You would be well advised to keep your eyes and ears open looking for a solution to problems that real people are having. When you decide what widget or service you want to provide, then you need some sort of business plan. This doesn't need to be a formal written one (although if you want bank financing it will need to be), but it must give a detailed plan of how you want to do your venture. Who/how big is your client base, would they buy your product, how do you plan to advertise, how many can you/do you want to make, what is your TRUE production cost (99.99% of people starting a small business are both under capitalized and don't properly understand their cost of doing business. Fore example, if your markup for wholesale items is not at least 100%, you are probably going out of business soon).
It goes without saying, the thing you do has to be something you are the absolute best at. You almost have to apprentice for quite a while at anything that requires any amount of skill. Anything that you can do that doesn't require a specialized talent or skill will nt be very profitable 'cause by definition, anyone can do it.