The most common mistake people make when starting a new business is not thinking about what they're good at. Many people just think that they're going to start a business because they have an idea and then, somehow, everything will just work itself out. But that's not how it works.
The most successful businesses are successful because they have identified the areas where they have expertise and can provide value. If you are good at something, then you should focus on that thing! For instance, if you know how to make delicious peanut butter cookies, then start a cookie company! If you don't know how to make cookies but you love dogs, then start a dog-sitting service! You get the idea.